The successive formation constants for the complexation of copper(II) with poly(N-vinylimidazole) (PVIm) have been determined by means of spectral deconvolution. Based on these results, definitions of the successive formation constants for the complexation of transition metal ions with multidentate polymer ligands are proposed. These are K-1 = [CuL(*)]/[Cu][L(*)], K-2 = [CuL(2)(*)]/[CuL(*)], K-3 = [CuL(3)(*)]/[CuL(2)(*)], and K-4 = [CuL(4)(*)]/[CuL(3)(*)] (L(*) is ligand residue on the polymer, charge omitted). The definitions of K-2, K-3, and K-4 are quite unlike the generally used definition, and the difference is ascribed to the nature of a polymer chain, which makes a microheterogeneous region in the solution. The results indicate that the concentration ratio [CuL(*)]:[CuL(2)(*)] : [CuL(3)(*)]:[CuL(4)(*)] is constant when [L(*)] is varied. Present results seem to strongly suggest that the commonly used definitions of the successive formation constants for the complexation of transition metal ions with multidentate polymer ligands would be replaced by those proposed here, and re-examinations of the complexation of transition metal ions with multidentate polymer ligands may be necessary.