diff --git a/dlls/atl/atl_ax.c b/dlls/atl/atl_ax.c
index 2afc7460328..616713ce62b 100644
--- a/dlls/atl/atl_ax.c
+++ b/dlls/atl/atl_ax.c
@@ -950,7 +950,7 @@ HRESULT WINAPI AtlAxCreateControlEx(LPCOLESTR lpszName, HWND hWnd,
     TRACE("(%s %p %p %p %p %p %p)\n", debugstr_w(lpszName), hWnd, pStream, 
             ppUnkContainer, ppUnkControl, iidSink, punkSink);
 
-    hRes = CLSIDFromString( (LPOLESTR) lpszName, &controlId );
+    hRes = CLSIDFromString( lpszName, &controlId );
     if ( FAILED(hRes) )
         hRes = CLSIDFromProgID( lpszName, &controlId );
     if ( SUCCEEDED( hRes ) )
diff --git a/dlls/msi/action.c b/dlls/msi/action.c
index cf403e19ab1..75bbc0944db 100644
--- a/dlls/msi/action.c
+++ b/dlls/msi/action.c
@@ -3098,7 +3098,7 @@ static UINT ITERATE_RegisterTypeLibraries(MSIRECORD *row, LPVOID param)
     {
         LPCWSTR guid;
         guid = MSI_RecordGetString(row,1);
-        CLSIDFromString((LPWSTR)guid, &tl_struct.clsid);
+        CLSIDFromString((LPCWSTR)guid, &tl_struct.clsid);
         tl_struct.source = strdupW( file->TargetPath );
         tl_struct.path = NULL;
 
@@ -3199,7 +3199,7 @@ static UINT ITERATE_UnregisterTypeLibraries( MSIRECORD *row, LPVOID param )
     ui_actiondata( package, szUnregisterTypeLibraries, row );
 
     guid = MSI_RecordGetString( row, 1 );
-    CLSIDFromString( (LPWSTR)guid, &libid );
+    CLSIDFromString( (LPCWSTR)guid, &libid );
     version = MSI_RecordGetInteger( row, 4 );
     language = MSI_RecordGetInteger( row, 2 );
 
diff --git a/dlls/msi/registry.c b/dlls/msi/registry.c
index 903e841cd7a..3e03ecf7c65 100644
--- a/dlls/msi/registry.c
+++ b/dlls/msi/registry.c
@@ -286,7 +286,7 @@ BOOL squash_guid(LPCWSTR in, LPWSTR out)
 
     out[0] = 0;
 
-    if (FAILED(CLSIDFromString((LPOLESTR)in, &guid)))
+    if (FAILED(CLSIDFromString((LPCOLESTR)in, &guid)))
         return FALSE;
 
     for(i=0; i<8; i++)
diff --git a/dlls/msi/tests/msi.c b/dlls/msi/tests/msi.c
index 428f4f507fe..4bc59ac041b 100644
--- a/dlls/msi/tests/msi.c
+++ b/dlls/msi/tests/msi.c
@@ -467,7 +467,7 @@ static BOOL squash_guid(LPCWSTR in, LPWSTR out)
     DWORD i,n=1;
     GUID guid;
 
-    if (FAILED(CLSIDFromString((LPOLESTR)in, &guid)))
+    if (FAILED(CLSIDFromString((LPCOLESTR)in, &guid)))
         return FALSE;
 
     for(i=0; i<8; i++)
diff --git a/dlls/msi/tests/package.c b/dlls/msi/tests/package.c
index ddb3522e94d..e2a95c09e35 100644
--- a/dlls/msi/tests/package.c
+++ b/dlls/msi/tests/package.c
@@ -153,7 +153,7 @@ static BOOL squash_guid(LPCWSTR in, LPWSTR out)
     DWORD i,n=1;
     GUID guid;
 
-    if (FAILED(CLSIDFromString((LPOLESTR)in, &guid)))
+    if (FAILED(CLSIDFromString((LPCOLESTR)in, &guid)))
         return FALSE;
 
     for(i=0; i<8; i++)
diff --git a/dlls/msi/tests/source.c b/dlls/msi/tests/source.c
index fd983ae50f0..1826f2175be 100644
--- a/dlls/msi/tests/source.c
+++ b/dlls/msi/tests/source.c
@@ -81,7 +81,7 @@ static BOOL squash_guid(LPCWSTR in, LPWSTR out)
     DWORD i,n=1;
     GUID guid;
 
-    if (FAILED(CLSIDFromString((LPOLESTR)in, &guid)))
+    if (FAILED(CLSIDFromString((LPCOLESTR)in, &guid)))
         return FALSE;
 
     for(i=0; i<8; i++)
diff --git a/dlls/ole32/compobj.c b/dlls/ole32/compobj.c
index 5e010b1c310..0fb074dc50c 100644
--- a/dlls/ole32/compobj.c
+++ b/dlls/ole32/compobj.c
@@ -1506,7 +1506,7 @@ HRESULT WINAPI CoCreateGuid(GUID *pguid)
  * SEE ALSO
  *  StringFromCLSID
  */
-static HRESULT __CLSIDFromString(LPCWSTR s, CLSID *id)
+static HRESULT __CLSIDFromString(LPCWSTR s, LPCLSID id)
 {
   int	i;
   BYTE table[256];
@@ -1566,7 +1566,7 @@ static HRESULT __CLSIDFromString(LPCWSTR s, CLSID *id)
 
 /*****************************************************************************/
 
-HRESULT WINAPI CLSIDFromString(LPOLESTR idstr, CLSID *id )
+HRESULT WINAPI CLSIDFromString(LPCOLESTR idstr, LPCLSID id )
 {
     HRESULT ret;
 
diff --git a/dlls/shell32/shellole.c b/dlls/shell32/shellole.c
index 8d09d9cf39e..dc4a3b39719 100644
--- a/dlls/shell32/shellole.c
+++ b/dlls/shell32/shellole.c
@@ -252,7 +252,7 @@ DWORD WINAPI SHCLSIDFromStringA (LPCSTR clsid, CLSID *id)
 DWORD WINAPI SHCLSIDFromStringW (LPCWSTR clsid, CLSID *id)
 {
 	TRACE("(%p(%s) %p)\n", clsid, debugstr_w(clsid), id);
-	return CLSIDFromString((LPWSTR)clsid, id);
+	return CLSIDFromString(clsid, id);
 }
 DWORD WINAPI SHCLSIDFromStringAW (LPCVOID clsid, CLSID *id)
 {
diff --git a/dlls/shlwapi/ordinal.c b/dlls/shlwapi/ordinal.c
index 8f3e29cc2c2..82092176ecc 100644
--- a/dlls/shlwapi/ordinal.c
+++ b/dlls/shlwapi/ordinal.c
@@ -2818,7 +2818,7 @@ BOOL WINAPI GUIDFromStringA(LPCSTR idstr, CLSID *id)
  */
 BOOL WINAPI GUIDFromStringW(LPCWSTR idstr, CLSID *id)
 {
-    return SUCCEEDED(CLSIDFromString((LPOLESTR)idstr, id));
+    return SUCCEEDED(CLSIDFromString((LPCOLESTR)idstr, id));
 }
 
 /*************************************************************************
@@ -3903,7 +3903,7 @@ DWORD WINAPI SHSendMessageBroadcastW(UINT uMsg, WPARAM wParam, LPARAM lParam)
  */
 HRESULT WINAPI CLSIDFromStringWrap(LPCWSTR idstr, CLSID *id)
 {
-    return CLSIDFromString((LPOLESTR)idstr, id);
+    return CLSIDFromString((LPCOLESTR)idstr, id);
 }
 
 /*************************************************************************
diff --git a/dlls/urlmon/tests/url.c b/dlls/urlmon/tests/url.c
index d5b9d125e9e..09cbb424239 100644
--- a/dlls/urlmon/tests/url.c
+++ b/dlls/urlmon/tests/url.c
@@ -1388,7 +1388,7 @@ static HRESULT WINAPI statusclb_OnProgress(IBindStatusCallbackEx *iface, ULONG u
             CHECK_EXPECT(Obj_OnProgress_CLASSIDAVAILABLE);
         else
             todo_wine CHECK_EXPECT(Obj_OnProgress_CLASSIDAVAILABLE);
-        hr = CLSIDFromString((LPOLESTR)szStatusText, &clsid);
+        hr = CLSIDFromString((LPCOLESTR)szStatusText, &clsid);
         ok(hr == S_OK, "CLSIDFromString failed with error 0x%08x\n", hr);
         ok(IsEqualCLSID(&clsid, &CLSID_HTMLDocument),
             "Expected clsid to be CLSID_HTMLDocument instead of %s\n", debugstr_guid(&clsid));
diff --git a/include/objbase.h b/include/objbase.h
index b9f50ab8e08..52c6b56cef8 100644
--- a/include/objbase.h
+++ b/include/objbase.h
@@ -431,7 +431,7 @@ HRESULT WINAPI CoWaitForMultipleHandles(DWORD dwFlags,DWORD dwTimeout,ULONG cHan
  *	GUID API
  */
 HRESULT WINAPI StringFromCLSID(REFCLSID id, LPOLESTR*);
-HRESULT WINAPI CLSIDFromString(LPOLESTR, CLSID *);
+HRESULT WINAPI CLSIDFromString(LPCOLESTR, LPCLSID);
 HRESULT WINAPI CLSIDFromProgID(LPCOLESTR progid, LPCLSID riid);
 HRESULT WINAPI ProgIDFromCLSID(REFCLSID clsid, LPOLESTR *lplpszProgID);