A composition comprising a source of metal ions and at least one leveling agent comprising a linear or branched, polymeric imidazolium compound comprising the structural unit of formula L1 (L1) wherein R1, R2, R3 are each independently selected from an H atom and an organic radical having from 1 to 20 carbon atoms, R4 is a divalent, trivalent or mutlivalent organic radical which does not comprise a hydroxyl group in the α or β position relative to the nitrogen atom of the imidazole rings is an integer.