diff --git a/dlls/mp3dmod/mp3dmod.c b/dlls/mp3dmod/mp3dmod.c
index 5e232f0443f..6a904716bc1 100644
--- a/dlls/mp3dmod/mp3dmod.c
+++ b/dlls/mp3dmod/mp3dmod.c
@@ -325,9 +325,16 @@ static HRESULT WINAPI MediaObject_GetInputSizeInfo(IMediaObject *iface,
 
 static HRESULT WINAPI MediaObject_GetOutputSizeInfo(IMediaObject *iface, DWORD index, DWORD *size, DWORD *alignment)
 {
-    FIXME("(%p)->(%d, %p, %p) stub!\n", iface, index, size, alignment);
+    struct mp3_decoder *dmo = impl_from_IMediaObject(iface);
 
-    return E_NOTIMPL;
+    TRACE("iface %p, index %u, size %p, alignment %p.\n", iface, index, size, alignment);
+
+    if (!dmo->intype_set || !dmo->outtype_set)
+        return DMO_E_TYPE_NOT_SET;
+
+    *size = 2 * 1152 * ((WAVEFORMATEX *)dmo->outtype.pbFormat)->wBitsPerSample / 8;
+    *alignment = 1;
+    return S_OK;
 }
 
 static HRESULT WINAPI MediaObject_GetInputMaxLatency(IMediaObject *iface, DWORD index, REFERENCE_TIME *latency)
diff --git a/dlls/mp3dmod/tests/mp3dmod.c b/dlls/mp3dmod/tests/mp3dmod.c
index 78471c42bcb..285b76023b0 100644
--- a/dlls/mp3dmod/tests/mp3dmod.c
+++ b/dlls/mp3dmod/tests/mp3dmod.c
@@ -342,12 +342,16 @@ static void test_stream_info(void)
 
     hr = IMediaObject_GetInputSizeInfo(dmo, 0, &size, &lookahead, &alignment);
     ok(hr == DMO_E_TYPE_NOT_SET, "Got hr %#x.\n", hr);
+    hr = IMediaObject_GetOutputSizeInfo(dmo, 0, &size, &alignment);
+    ok(hr == DMO_E_TYPE_NOT_SET, "Got hr %#x.\n", hr);
 
     hr = IMediaObject_SetInputType(dmo, 0, &input_mt, 0);
     ok(hr == S_OK, "Got hr %#x.\n", hr);
 
     hr = IMediaObject_GetInputSizeInfo(dmo, 0, &size, &lookahead, &alignment);
     ok(hr == DMO_E_TYPE_NOT_SET, "Got hr %#x.\n", hr);
+    hr = IMediaObject_GetOutputSizeInfo(dmo, 0, &size, &alignment);
+    ok(hr == DMO_E_TYPE_NOT_SET, "Got hr %#x.\n", hr);
 
     hr = IMediaObject_SetOutputType(dmo, 0, &output_mt, 0);
     ok(hr == S_OK, "Got hr %#x.\n", hr);
@@ -359,6 +363,12 @@ static void test_stream_info(void)
     ok(lookahead == 0xdeadbeef, "Got lookahead %u.\n", lookahead);
     ok(alignment == 1, "Got alignment %u.\n", alignment);
 
+    size = alignment = 0xdeadbeef;
+    hr = IMediaObject_GetOutputSizeInfo(dmo, 0, &size, &alignment);
+    ok(hr == S_OK, "Got hr %#x.\n", hr);
+    ok(size == 1152 * 4, "Got size %u.\n", size);
+    ok(alignment == 1, "Got alignment %u.\n", alignment);
+
     IMediaObject_Release(dmo);
 }