In the C2HS.hs which is generated by 'c2hs -l', I found this code: instance Storable a => Storable (Maybe a) where sizeOf _ = sizeOf (undefined :: Ptr ()) alignment _ = alignment (undefined :: Ptr ()) peek p = do ptr <- peek (castPtr p) if ptr == nullPtr then return Nothing else liftM Just $ peek ptr poke p v = do ptr <- case v of Nothing -> return nullPtr Just v' -> new v' poke (castPtr p) ptr In the poke implementation, 'Foreign.Marshal.Utils.new' is used. 'new' uses malloc, and would thus most likely lead to memory leaks, since the user of this instance probably doesn't know that he needs to free the block pointed to by the written value. Or did I miss something clever here? The best alternative I can see is to restrict it to Storable a => Storable (Maybe (Ptr a)), and skip the call to 'new', but that requires extensions. /Björn