diff --git a/services/acl/Acl.Api/Program.cs b/services/acl/Acl.Api/Program.cs
index f220cb1..9581455 100644
--- a/services/acl/Acl.Api/Program.cs
+++ b/services/acl/Acl.Api/Program.cs
@@ -40,6 +40,15 @@ app.MapPost("/zaken/reference", async (ZaakReferenceRequest body, AclService acl
     return Results.Ok(new { reference });
 });
 
+// Store an uploaded diploma against a zaak (S-10b): the domain sends the file as base64; the ACL
+// creates the ZGW enkelvoudiginformatieobject and relates it to the zaak (§8.1). Returns its URL.
+app.MapPost("/documenten", async (StoreDocumentRequest body, AclService acl, CancellationToken ct) =>
+{
+    var url = await acl.StoreDiplomaAsync(
+        new Uri(body.ZaakUrl), Convert.FromBase64String(body.ContentBase64), body.FileName, body.ContentType, ct);
+    return Results.Ok(new { informatieobjectUrl = url.ToString() });
+});
+
 app.Run();
 
 public sealed record OpenZaakRequest(string Bsn, string Reference);
@@ -48,4 +57,6 @@ public sealed record SetStatusRequest(string ZaakUrl);
 
 public sealed record ZaakReferenceRequest(string ZaakUrl);
 
+public sealed record StoreDocumentRequest(string ZaakUrl, string ContentBase64, string FileName, string ContentType);
+
 public partial class Program;
diff --git a/services/acl/Acl.Application/AclDefaults.cs b/services/acl/Acl.Application/AclDefaults.cs
index 232d3a4..d378c8c 100644
--- a/services/acl/Acl.Application/AclDefaults.cs
+++ b/services/acl/Acl.Application/AclDefaults.cs
@@ -7,4 +7,8 @@ public sealed class AclDefaults
     public required string VerantwoordelijkeOrganisatie { get; init; }
     public required string Vertrouwelijkheidaanduiding { get; init; }
     public required Uri ZaaktypeUrl { get; init; }
+
+    /// <summary>The informatieobjecttype an uploaded diploma is filed under (S-10b). Seeded in the
+    /// catalogus and injected like <see cref="ZaaktypeUrl"/>.</summary>
+    public required Uri InformatieobjecttypeUrl { get; init; }
 }
diff --git a/services/acl/Acl.Application/AclService.cs b/services/acl/Acl.Application/AclService.cs
index a70e7ca..f692041 100644
--- a/services/acl/Acl.Application/AclService.cs
+++ b/services/acl/Acl.Application/AclService.cs
@@ -37,4 +37,33 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, ICloc
 
         return gateway.GetZaakIdentificatieAsync(zaakUrl, ct);
     }
+
+    /// <summary>
+    /// Store an uploaded diploma against the zaak (S-10b): default-fill the ZGW-mandatory document
+    /// fields (informatieobjecttype, bronorganisatie, vertrouwelijkheidaanduiding, taal, creatiedatum)
+    /// and hand the file to the gateway, which creates the informatieobject and relates it to the zaak.
+    /// The domain supplies only the zaak, the bytes, and the file's name/type (§8.1).
+    /// </summary>
+    public Task<Uri> StoreDiplomaAsync(Uri zaakUrl, byte[] content, string fileName, string contentType, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+        ArgumentNullException.ThrowIfNull(content);
+        ArgumentException.ThrowIfNullOrWhiteSpace(fileName);
+        ArgumentException.ThrowIfNullOrWhiteSpace(contentType);
+
+        var request = new DocumentRequest(
+            defaults.Bronorganisatie,
+            defaults.InformatieobjecttypeUrl,
+            defaults.Vertrouwelijkheidaanduiding,
+            zaakUrl,
+            clock.Today,
+            Titel: "Diploma",
+            Auteur: "zorgprofessional",
+            Taal: "nld",
+            Bestandsnaam: fileName,
+            Formaat: contentType,
+            Inhoud: content);
+
+        return gateway.StoreDocumentAsync(request, ct);
+    }
 }
diff --git a/services/acl/Acl.Application/DocumentRequest.cs b/services/acl/Acl.Application/DocumentRequest.cs
new file mode 100644
index 0000000..885148f
--- /dev/null
+++ b/services/acl/Acl.Application/DocumentRequest.cs
@@ -0,0 +1,17 @@
+namespace Acl.Application;
+
+/// <summary>The fully default-filled diploma document the gateway will create in the ZGW Documenten
+/// API and relate to the zaak (S-10b). <see cref="Inhoud"/> is the raw file content; the gateway
+/// base64-encodes it into the ZGW <c>inhoud</c> field.</summary>
+public sealed record DocumentRequest(
+    string Bronorganisatie,
+    Uri Informatieobjecttype,
+    string Vertrouwelijkheidaanduiding,
+    Uri Zaak,
+    DateOnly Creatiedatum,
+    string Titel,
+    string Auteur,
+    string Taal,
+    string Bestandsnaam,
+    string Formaat,
+    byte[] Inhoud);
diff --git a/services/acl/Acl.Application/IZaakGateway.cs b/services/acl/Acl.Application/IZaakGateway.cs
index 583baf5..73903f0 100644
--- a/services/acl/Acl.Application/IZaakGateway.cs
+++ b/services/acl/Acl.Application/IZaakGateway.cs
@@ -16,4 +16,11 @@ public interface IZaakGateway
     /// <summary>Read the zaak's <c>identificatie</c> — the public-safe reference the register shows.
     /// The Event Subscriber calls this through the ACL rather than reading ZGW itself (§8.1, #78).</summary>
     Task<string> GetZaakIdentificatieAsync(Uri zaakUrl, CancellationToken ct = default);
+
+    /// <summary>
+    /// Store a diploma document (S-10b): create an <c>enkelvoudiginformatieobject</c> in the ZGW
+    /// Documenten API and relate it to the zaak via a <c>zaakinformatieobject</c>. Returns the URL of
+    /// the created informatieobject.
+    /// </summary>
+    Task<Uri> StoreDocumentAsync(DocumentRequest request, CancellationToken ct = default);
 }
diff --git a/services/acl/Acl.Infrastructure/OpenZaakGateway.cs b/services/acl/Acl.Infrastructure/OpenZaakGateway.cs
index 92d85ea..cee0c9a 100644
--- a/services/acl/Acl.Infrastructure/OpenZaakGateway.cs
+++ b/services/acl/Acl.Infrastructure/OpenZaakGateway.cs
@@ -80,6 +80,36 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
         return zaak.Identificatie;
     }
 
+    public async Task<Uri> StoreDocumentAsync(DocumentRequest request, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        // 1. Create the enkelvoudiginformatieobject in the Documenten API (not a geo API — no CRS).
+        var created = await PostForUrlAsync(
+            "/documenten/api/v1/enkelvoudiginformatieobjecten",
+            new EnkelvoudigInformatieobjectDto(
+                request.Bronorganisatie,
+                request.Creatiedatum.ToString("yyyy-MM-dd"),
+                request.Titel,
+                request.Auteur,
+                request.Taal,
+                request.Informatieobjecttype.ToString(),
+                Convert.ToBase64String(request.Inhoud),
+                request.Bestandsnaam,
+                request.Inhoud.Length,
+                request.Vertrouwelijkheidaanduiding,
+                request.Formaat,
+                "definitief"),
+            "Creating the informatieobject", ct);
+
+        // 2. Relate it to the zaak (Zaken API — no CRS).
+        await PostAsync("/zaken/api/v1/zaakinformatieobjecten",
+            new ZaakInformatieobjectDto(request.Zaak.ToString(), created.ToString()),
+            "Relating the informatieobject to the zaak", ct);
+
+        return created;
+    }
+
     // POSTs a non-geo ZGW resource (resultaat/status — no CRS headers). Buffers the body so uwsgi gets
     // a Content-Length instead of a chunked body (as with zaak-create).
     private async Task PostAsync(string path, object dto, string action, CancellationToken ct)
@@ -96,6 +126,26 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
         await EnsureSuccessAsync(response, action, ct);
     }
 
+    // POSTs a non-geo ZGW resource and returns the created resource's URL (as PostAsync, but reads back
+    // the `url` of the created object). Buffers the body so uwsgi gets a Content-Length.
+    private async Task<Uri> PostForUrlAsync(string path, object dto, string action, CancellationToken ct)
+    {
+        using var message = new HttpRequestMessage(HttpMethod.Post, new Uri(options.BaseUrl, path))
+        {
+            Content = JsonContent.Create(dto),
+        };
+        message.Headers.Authorization =
+            new AuthenticationHeaderValue("Bearer", ZgwToken.Mint(options.ClientId, options.Secret));
+        await message.Content.LoadIntoBufferAsync(ct);
+
+        using var response = await http.SendAsync(message, ct);
+        await EnsureSuccessAsync(response, action, ct);
+
+        var created = await response.Content.ReadFromJsonAsync<CreatedDto>(ct)
+            ?? throw new InvalidOperationException($"OpenZaak returned an empty response for {action}");
+        return new Uri(created.Url);
+    }
+
     // EnsureSuccessStatusCode discards the response body; ZGW returns a JSON problem detail on 400 that
     // is essential for diagnosing a rejected request, so surface it in the exception.
     private static async Task EnsureSuccessAsync(HttpResponseMessage response, string action, CancellationToken ct)
@@ -182,4 +232,25 @@ public sealed class OpenZaakGateway(HttpClient http, OpenZaakOptions options) :
 
     private sealed record ResultaattypeDto(
         [property: JsonPropertyName("url")] string Url);
+
+    private sealed record CreatedDto(
+        [property: JsonPropertyName("url")] string Url);
+
+    private sealed record EnkelvoudigInformatieobjectDto(
+        [property: JsonPropertyName("bronorganisatie")] string Bronorganisatie,
+        [property: JsonPropertyName("creatiedatum")] string Creatiedatum,
+        [property: JsonPropertyName("titel")] string Titel,
+        [property: JsonPropertyName("auteur")] string Auteur,
+        [property: JsonPropertyName("taal")] string Taal,
+        [property: JsonPropertyName("informatieobjecttype")] string Informatieobjecttype,
+        [property: JsonPropertyName("inhoud")] string Inhoud,
+        [property: JsonPropertyName("bestandsnaam")] string Bestandsnaam,
+        [property: JsonPropertyName("bestandsomvang")] int Bestandsomvang,
+        [property: JsonPropertyName("vertrouwelijkheidaanduiding")] string Vertrouwelijkheidaanduiding,
+        [property: JsonPropertyName("formaat")] string Formaat,
+        [property: JsonPropertyName("status")] string Status);
+
+    private sealed record ZaakInformatieobjectDto(
+        [property: JsonPropertyName("zaak")] string Zaak,
+        [property: JsonPropertyName("informatieobject")] string Informatieobject);
 }
diff --git a/services/acl/Acl.Tests/AclServiceTests.cs b/services/acl/Acl.Tests/AclServiceTests.cs
index 2e74c8f..fe2a81a 100644
--- a/services/acl/Acl.Tests/AclServiceTests.cs
+++ b/services/acl/Acl.Tests/AclServiceTests.cs
@@ -30,6 +30,15 @@ public class AclServiceTests
             ReadReferenceFor = zaakUrl;
             return Task.FromResult("REG-FROM-ZAAK");
         }
+
+        public DocumentRequest? StoredDocument;
+        public Uri DocumentResult { get; } = new("http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1");
+
+        public Task<Uri> StoreDocumentAsync(DocumentRequest request, CancellationToken ct = default)
+        {
+            StoredDocument = request;
+            return Task.FromResult(DocumentResult);
+        }
     }
 
     private static AclDefaults Defaults() => new()
@@ -38,6 +47,7 @@ public class AclServiceTests
         VerantwoordelijkeOrganisatie = "517439943",
         Vertrouwelijkheidaanduiding = "openbaar",
         ZaaktypeUrl = new("http://openzaak/catalogi/api/v1/zaaktypen/big"),
+        InformatieobjecttypeUrl = new("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip"),
     };
 
     private sealed class FixedClock(DateOnly today) : IClock
@@ -55,6 +65,7 @@ public class AclServiceTests
             VerantwoordelijkeOrganisatie = "517439943",
             Vertrouwelijkheidaanduiding = "openbaar",
             ZaaktypeUrl = new("http://openzaak/catalogi/api/v1/zaaktypen/big"),
+            InformatieobjecttypeUrl = new("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip"),
         };
         var service = new AclService(gateway, defaults, new FixedClock(new DateOnly(2026, 6, 4)));
 
@@ -81,6 +92,7 @@ public class AclServiceTests
             VerantwoordelijkeOrganisatie = "517439943",
             Vertrouwelijkheidaanduiding = "openbaar",
             ZaaktypeUrl = new("http://openzaak/catalogi/api/v1/zaaktypen/big"),
+            InformatieobjecttypeUrl = new("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip"),
         };
         var service = new AclService(gateway, defaults, new FixedClock(new DateOnly(2026, 6, 4)));
 
@@ -114,6 +126,41 @@ public class AclServiceTests
         Assert.Null(gateway.Approved);
     }
 
+    [Fact]
+    public async Task Storing_a_diploma_default_fills_the_document_fields_and_returns_its_url()
+    {
+        var gateway = new FakeGateway();
+        var defaults = Defaults();
+        var service = new AclService(gateway, defaults, new FixedClock(new DateOnly(2026, 6, 4)));
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        var url = await service.StoreDiplomaAsync(zaak, [1, 2, 3], "diploma.pdf", "application/pdf");
+
+        Assert.Equal(gateway.DocumentResult, url);
+        var req = gateway.StoredDocument!;
+        Assert.Equal(zaak, req.Zaak);
+        Assert.Equal(defaults.InformatieobjecttypeUrl, req.Informatieobjecttype);
+        Assert.Equal("517439943", req.Bronorganisatie);
+        Assert.Equal("openbaar", req.Vertrouwelijkheidaanduiding);
+        Assert.Equal(new DateOnly(2026, 6, 4), req.Creatiedatum);
+        Assert.Equal("nld", req.Taal);
+        Assert.Equal("diploma.pdf", req.Bestandsnaam);
+        Assert.Equal("application/pdf", req.Formaat);
+        Assert.Equal(new byte[] { 1, 2, 3 }, req.Inhoud);
+    }
+
+    [Fact]
+    public async Task Storing_a_diploma_rejects_null_or_blank_arguments()
+    {
+        var service = new AclService(new FakeGateway(), Defaults(), new FixedClock(new DateOnly(2026, 6, 4)));
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => service.StoreDiplomaAsync(null!, [1], "d.pdf", "application/pdf"));
+        await Assert.ThrowsAsync<ArgumentNullException>(() => service.StoreDiplomaAsync(zaak, null!, "d.pdf", "application/pdf"));
+        await Assert.ThrowsAnyAsync<ArgumentException>(() => service.StoreDiplomaAsync(zaak, [1], " ", "application/pdf"));
+        await Assert.ThrowsAnyAsync<ArgumentException>(() => service.StoreDiplomaAsync(zaak, [1], "d.pdf", " "));
+    }
+
     [Fact]
     public async Task Reading_a_zaak_reference_returns_the_zaaks_identificatie()
     {