diff --git a/bridge.cpp b/bridge.cpp
index 176d1b53a0..9db92a601a 100644
--- a/bridge.cpp
+++ b/bridge.cpp
@@ -218,10 +218,10 @@ QList< ClientGroupItem > Bridge::clientGroupItems() const
     return c->clientGroup()->items();
     }
 
-int Bridge::itemId( int index )
+long Bridge::itemId( int index )
     {
     const ClientList list = c->clientGroup()->clients();
-    return reinterpret_cast<int>( list.at( index ));
+    return reinterpret_cast<long>( list.at( index ));
     }
 
 int Bridge::visibleClientGroupItem()
@@ -239,7 +239,7 @@ void Bridge::moveItemInClientGroup( int index, int before )
     c->clientGroup()->move( index, before );
     }
 
-void Bridge::moveItemToClientGroup( int itemId, int before )
+void Bridge::moveItemToClientGroup( long itemId, int before )
     {
     Client* item = reinterpret_cast<Client*>( itemId );
     c->workspace()->moveItemToClientGroup( item->clientGroup(), item->clientGroup()->indexOfClient( item ),
diff --git a/bridge.h b/bridge.h
index 169f34d843..b467e8f6bf 100644
--- a/bridge.h
+++ b/bridge.h
@@ -80,11 +80,11 @@ class Bridge : public KDecorationBridgeUnstable
         // Window tabbing
         virtual bool isClientGroupActive();
         virtual QList< ClientGroupItem > clientGroupItems() const;
-        virtual int itemId( int index );
+        virtual long itemId( int index );
         virtual int visibleClientGroupItem();
         virtual void setVisibleClientGroupItem( int index );
         virtual void moveItemInClientGroup( int index, int before );
-        virtual void moveItemToClientGroup( int itemId, int before );
+        virtual void moveItemToClientGroup( long itemId, int before );
         virtual void removeFromClientGroup( int index, const QRect& newGeom );
         virtual void closeClientGroupItem( int index );
         virtual void closeAllInClientGroup();
diff --git a/clients/oxygen/oxygenclient.cpp b/clients/oxygen/oxygenclient.cpp
index 68e4255582..a78d63bd5b 100644
--- a/clients/oxygen/oxygenclient.cpp
+++ b/clients/oxygen/oxygenclient.cpp
@@ -1616,7 +1616,7 @@ namespace Oxygen
 
       setForceActive( true );
       int itemClicked( OxygenClient::itemClicked( point, true ) );
-      int source = QString( groupData->data( clientGroupItemDragMimeType() ) ).toInt();
+      long source = QString( groupData->data( clientGroupItemDragMimeType() ) ).toLong();
       moveItemToClientGroup( source, itemClicked );
 
     }
diff --git a/clients/tabstrip/tabstripdecoration.cpp b/clients/tabstrip/tabstripdecoration.cpp
index ee8df3c7c7..82bace1b09 100644
--- a/clients/tabstrip/tabstripdecoration.cpp
+++ b/clients/tabstrip/tabstripdecoration.cpp
@@ -380,7 +380,7 @@ bool TabstripDecoration::dropEvent( QDropEvent* e )
                 }
             else
                 {
-                int source = QString( group_data->data( clientGroupItemDragMimeType() ) ).toInt();
+                long source = QString( group_data->data( clientGroupItemDragMimeType() ) ).toLong();
                 moveItemToClientGroup( source, itemClicked( point, true ));
                 }
             return true;
diff --git a/kcmkwin/kwindecoration/preview.cpp b/kcmkwin/kwindecoration/preview.cpp
index 058426d529..8ed5d8fdf7 100644
--- a/kcmkwin/kwindecoration/preview.cpp
+++ b/kcmkwin/kwindecoration/preview.cpp
@@ -480,7 +480,7 @@ QList< ClientGroupItem > KDecorationPreviewBridge::clientGroupItems() const
         active ? "Active Window" : "Inactive Window", icon() );
     }
 
-int KDecorationPreviewBridge::itemId( int )
+long KDecorationPreviewBridge::itemId( int )
     {
     return 0;
     }
@@ -498,7 +498,7 @@ void KDecorationPreviewBridge::moveItemInClientGroup( int, int )
     {
     }
 
-void KDecorationPreviewBridge::moveItemToClientGroup( int, int )
+void KDecorationPreviewBridge::moveItemToClientGroup( long, int )
     {
     }
 
diff --git a/kcmkwin/kwindecoration/preview.h b/kcmkwin/kwindecoration/preview.h
index cee3d03d09..7ebbdb2990 100644
--- a/kcmkwin/kwindecoration/preview.h
+++ b/kcmkwin/kwindecoration/preview.h
@@ -119,11 +119,11 @@ class KDecorationPreviewBridge
         // Window tabbing
         virtual bool isClientGroupActive();
         virtual QList< ClientGroupItem > clientGroupItems() const;
-        virtual int itemId( int index );
+        virtual long itemId( int index );
         virtual int visibleClientGroupItem();
         virtual void setVisibleClientGroupItem( int index );
         virtual void moveItemInClientGroup( int index, int before );
-        virtual void moveItemToClientGroup( int itemId, int before );
+        virtual void moveItemToClientGroup( long itemId, int before );
         virtual void removeFromClientGroup( int index, const QRect& newGeom );
         virtual void closeClientGroupItem( int index );
         virtual void closeAllInClientGroup();
diff --git a/lib/kcommondecoration.cpp b/lib/kcommondecoration.cpp
index 470948bcd5..c62283ebd4 100644
--- a/lib/kcommondecoration.cpp
+++ b/lib/kcommondecoration.cpp
@@ -1291,7 +1291,7 @@ QList< ClientGroupItem > KCommonDecorationUnstable::clientGroupItems() const
     return static_cast<const KDecorationUnstable*>( decoration() )->clientGroupItems();
     }
 
-int KCommonDecorationUnstable::itemId( int index )
+long KCommonDecorationUnstable::itemId( int index )
     {
     return static_cast<const KDecorationUnstable*>( decoration() )->itemId( index );
     }
@@ -1311,7 +1311,7 @@ void KCommonDecorationUnstable::moveItemInClientGroup( int index, int before )
     static_cast<const KDecorationUnstable*>( decoration() )->moveItemInClientGroup( index, before );
     }
 
-void KCommonDecorationUnstable::moveItemToClientGroup( int itemId, int before )
+void KCommonDecorationUnstable::moveItemToClientGroup( long itemId, int before )
     {
     static_cast<const KDecorationUnstable*>( decoration() )->moveItemToClientGroup( itemId, before );
     }
diff --git a/lib/kcommondecoration.h b/lib/kcommondecoration.h
index 0af3385923..815186a4ec 100644
--- a/lib/kcommondecoration.h
+++ b/lib/kcommondecoration.h
@@ -384,11 +384,11 @@ class KWIN_EXPORT KCommonDecorationUnstable
         // Window tabbing
         bool isClientGroupActive();
         QList< ClientGroupItem > clientGroupItems() const;
-        int itemId( int index );
+        long itemId( int index );
         int visibleClientGroupItem();
         void setVisibleClientGroupItem( int index );
         void moveItemInClientGroup( int index, int before );
-        void moveItemToClientGroup( int itemId, int before = -1 );
+        void moveItemToClientGroup( long itemId, int before = -1 );
         void removeFromClientGroup( int index, const QRect& newGeom = QRect() );
         void closeClientGroupItem( int index );
         void closeAllInClientGroup();
diff --git a/lib/kdecoration.cpp b/lib/kdecoration.cpp
index b2d53b6746..99e25116fe 100644
--- a/lib/kdecoration.cpp
+++ b/lib/kdecoration.cpp
@@ -415,7 +415,7 @@ QList< ClientGroupItem > KDecorationUnstable::clientGroupItems() const
     return static_cast< KDecorationBridgeUnstable* >( bridge_ )->clientGroupItems();
     }
 
-int KDecorationUnstable::itemId( int index )
+long KDecorationUnstable::itemId( int index )
     {
     return static_cast< KDecorationBridgeUnstable* >( bridge_ )->itemId( index );
     }
@@ -435,7 +435,7 @@ void KDecorationUnstable::moveItemInClientGroup( int index, int before )
     static_cast< KDecorationBridgeUnstable* >( bridge_ )->moveItemInClientGroup( index, before );
     }
 
-void KDecorationUnstable::moveItemToClientGroup( int itemId, int before )
+void KDecorationUnstable::moveItemToClientGroup( long itemId, int before )
     {
     static_cast< KDecorationBridgeUnstable* >( bridge_ )->moveItemToClientGroup( itemId, before );
     }
diff --git a/lib/kdecoration.h b/lib/kdecoration.h
index 4ee173f32f..e306c5232f 100644
--- a/lib/kdecoration.h
+++ b/lib/kdecoration.h
@@ -939,7 +939,7 @@ class KWIN_EXPORT KDecorationUnstable
          * Returns a unique identifier for the client at index \p index of the client group list.
          * \see moveItemToClientGroup()
          */
-        int itemId( int index );
+        long itemId( int index );
         /**
          * Returns the list index of the currently visible client in this group.
          */
@@ -959,7 +959,7 @@ class KWIN_EXPORT KDecorationUnstable
          * moveItemInClientGroup() instead.
          * \see itemId()
          */
-        void moveItemToClientGroup( int itemId, int before = -1 );
+        void moveItemToClientGroup( long itemId, int before = -1 );
         /**
          * Remove the client at index \p index from the group. If \p newGeom is set then the client
          * will move and resize to the specified geometry, otherwise it will stay where the group
diff --git a/lib/kdecorationbridge.h b/lib/kdecorationbridge.h
index fb48f88b9d..ef7b73894a 100644
--- a/lib/kdecorationbridge.h
+++ b/lib/kdecorationbridge.h
@@ -98,11 +98,11 @@ class KWIN_EXPORT KDecorationBridgeUnstable
         // Window tabbing
         virtual bool isClientGroupActive() = 0;
         virtual QList< ClientGroupItem > clientGroupItems() const = 0;
-        virtual int itemId( int index ) = 0;
+        virtual long itemId( int index ) = 0;
         virtual int visibleClientGroupItem() = 0;
         virtual void setVisibleClientGroupItem( int index ) = 0;
         virtual void moveItemInClientGroup( int index, int before ) = 0;
-        virtual void moveItemToClientGroup( int itemId, int before ) = 0;
+        virtual void moveItemToClientGroup( long itemId, int before ) = 0;
         virtual void removeFromClientGroup( int index, const QRect& newGeom ) = 0;
         virtual void closeClientGroupItem( int index ) = 0;
         virtual void closeAllInClientGroup() = 0;
diff --git a/sm.cpp b/sm.cpp
index c28a7a0282..7404fafd38 100644
--- a/sm.cpp
+++ b/sm.cpp
@@ -129,7 +129,9 @@ void Workspace::storeSession( KConfig* config, SMSavePhase phase )
             cg.writeEntry( QString("stackingOrder")+n, unconstrained_stacking_order.indexOf( c ));
             int group = 0;
             if( c->clientGroup() )
-                group = c->clientGroup()->clients().count() > 1 ? (int) c->clientGroup() : 0;
+                group = c->clientGroup()->clients().count() > 1 ?
+                    // KConfig doesn't support long so we need to live with less precision on 64-bit systems
+                    static_cast<int>( reinterpret_cast<long>( c->clientGroup() )) : 0;
             cg.writeEntry( QString("clientGroup")+n, group );
             }
         }