diff --git a/services/acl/Acl.Tests/OpenZaakGatewayTests.cs b/services/acl/Acl.Tests/OpenZaakGatewayTests.cs
index af568c8..e5974b1 100644
--- a/services/acl/Acl.Tests/OpenZaakGatewayTests.cs
+++ b/services/acl/Acl.Tests/OpenZaakGatewayTests.cs
@@ -432,4 +432,111 @@ public class OpenZaakGatewayTests
         b64 = (b64.Length % 4) switch { 2 => b64 + "==", 3 => b64 + "=", _ => b64 };
         return Encoding.UTF8.GetString(Convert.FromBase64String(b64));
     }
+
+    // --- StoreDocumentAsync (diploma upload / S-10b) ---
+
+    private static readonly Uri Informatieobjecttype =
+        new("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip");
+
+    private static DocumentRequest SampleDocument(byte[]? inhoud = null) => new(
+        Bronorganisatie: "517439943",
+        Informatieobjecttype: Informatieobjecttype,
+        Vertrouwelijkheidaanduiding: "openbaar",
+        Zaak: new Uri(ZaakUrl),
+        Creatiedatum: new DateOnly(2026, 6, 4),
+        Titel: "Diploma",
+        Auteur: "zorgprofessional",
+        Taal: "nld",
+        Bestandsnaam: "diploma.pdf",
+        Formaat: "application/pdf",
+        Inhoud: inhoud ?? [1, 2, 3, 4]);
+
+    // Routes the two document calls: POST /enkelvoudiginformatieobjecten (documenten) then
+    // POST /zaakinformatieobjecten (zaken).
+    private static StubHandler DocumentStub(Recorder rec) => new(async req =>
+    {
+        rec.Requests.Add(req);
+        rec.ContentLengths.Add(req.Content?.Headers.ContentLength);
+        rec.Bodies.Add(req.Content is null ? null : await req.Content.ReadAsStringAsync());
+
+        return req.RequestUri!.ToString().Contains("/enkelvoudiginformatieobjecten")
+            ? Json(HttpStatusCode.Created, """{"url":"http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1"}""")
+            : Json(HttpStatusCode.Created, """{"url":"http://openzaak/zaken/api/v1/zaakinformatieobjecten/rel-1"}""");
+    });
+
+    [Fact]
+    public async Task Storing_a_document_creates_the_informatieobject_then_relates_it_to_the_zaak()
+    {
+        var rec = new Recorder();
+
+        var url = await Gateway(DocumentStub(rec)).StoreDocumentAsync(SampleDocument([10, 20, 30]));
+
+        Assert.Equal("http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1", url.ToString());
+
+        // 1. Create the enkelvoudiginformatieobject in the Documenten API.
+        var create = rec.Sent("/enkelvoudiginformatieobjecten");
+        Assert.Equal(HttpMethod.Post, create.Request.Method);
+        Assert.Equal("http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten",
+            create.Request.RequestUri!.ToString());
+        Assert.Equal("Bearer", create.Request.Headers.Authorization!.Scheme);
+        Assert.Contains("\"bronorganisatie\":\"517439943\"", create.Body);
+        Assert.Contains("\"informatieobjecttype\":\"http://openzaak/catalogi/api/v1/informatieobjecttypen/dip\"", create.Body);
+        Assert.Contains("\"creatiedatum\":\"2026-06-04\"", create.Body);
+        Assert.Contains("\"titel\":\"Diploma\"", create.Body);
+        Assert.Contains("\"auteur\":\"zorgprofessional\"", create.Body);
+        Assert.Contains("\"taal\":\"nld\"", create.Body);
+        Assert.Contains("\"bestandsnaam\":\"diploma.pdf\"", create.Body);
+        Assert.Contains("\"formaat\":\"application/pdf\"", create.Body);
+        Assert.Contains("\"vertrouwelijkheidaanduiding\":\"openbaar\"", create.Body);
+        Assert.Contains("\"status\":\"definitief\"", create.Body);
+        // The file content is base64-encoded into `inhoud`, with its byte length in `bestandsomvang`.
+        Assert.Contains($"\"inhoud\":\"{Convert.ToBase64String([10, 20, 30])}\"", create.Body);
+        Assert.Contains("\"bestandsomvang\":3", create.Body);
+
+        // 2. Relate that informatieobject to the zaak (Zaken API — no CRS).
+        var relate = rec.Sent("/zaakinformatieobjecten");
+        Assert.Equal(HttpMethod.Post, relate.Request.Method);
+        Assert.Equal("http://openzaak/zaken/api/v1/zaakinformatieobjecten",
+            relate.Request.RequestUri!.ToString());
+        Assert.Contains($"\"zaak\":\"{ZaakUrl}\"", relate.Body);
+        Assert.Contains("\"informatieobject\":\"http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1\"", relate.Body);
+    }
+
+    [Fact]
+    public async Task Storing_a_document_buffers_the_body_and_sends_no_crs_headers()
+    {
+        // uwsgi rejects a chunked body (Content-Length must be present); the Documenten API is not a
+        // geo API, so no CRS headers (unlike the Zaken zaak-create).
+        var rec = new Recorder();
+
+        await Gateway(DocumentStub(rec)).StoreDocumentAsync(SampleDocument());
+
+        var create = rec.Sent("/enkelvoudiginformatieobjecten");
+        Assert.NotNull(create.Length);
+        Assert.True(create.Length > 0);
+        Assert.False(create.Request.Headers.Contains("Accept-Crs"));
+        Assert.False(create.Request.Content!.Headers.Contains("Content-Crs"));
+    }
+
+    [Fact]
+    public async Task Storing_a_document_surfaces_an_openzaak_rejection()
+    {
+        var handler = new StubHandler(_ =>
+            Task.FromResult(new HttpResponseMessage(HttpStatusCode.BadRequest)
+            {
+                Content = new StringContent("""{"detail":"bad"}""", Encoding.UTF8, "application/json"),
+            }));
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(
+            () => Gateway(handler).StoreDocumentAsync(SampleDocument()));
+        Assert.Contains("bad", ex.Message);
+    }
+
+    [Fact]
+    public async Task Storing_a_document_rejects_a_null_request()
+    {
+        var handler = new StubHandler(_ => throw new InvalidOperationException("should not be sent"));
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => Gateway(handler).StoreDocumentAsync(null!));
+    }
 }