The effect of counterion types on the structure of Cu(II) complexes of imidazole and poly-4(5)-vinylimidazole in aqueous solution was studied by spectroscopic and electron spin resonance (ESR) techniques. Visible and ESR spectra of both complexes in aqueous solution containing NaClO4, KNO3, KC1, and KBr showed only a slight change from each other in the presence of different counterions. On the other hand, the frequency of the center of absorption band in the visible region (λmax) and ESR parameters of the complex in aqueous solution did not correspond to those reported for its crystalline state, and in addition, those of the polymeric Cu(II) complex were different from those of the low molecular weight Cu(II) complex. It was proposed that in aqueous solution, water molecules are coordinated to the axial position of the complex where counterion molecules are present in the crystalline state, and that the structure of the polymeric Cu(II) complex is different from that of the low molecular weight Cu(II) complex due to steric hindrance of the polymeric ligand.