The article discusses the formation of complexes between transition metal ions and a copolymer of crotonaldehyde with N-vinylpyrrolidone. Complexation of copolymers with metal ions in solutions has been studied by methods of IR-UV spectroscopy, sedimentation, diffusion and viscosimetry. Coordination of the metal with the copolymer occurs through the oxygen atom of the cycloamide and aldehyde groups.