Complexation of Cd(II) ions with polyacrylic acid has been studied potentiometrically and by Cd-113 NMR spectroscopy. A Donnan model proposed by Marinsky (Marinsky, J. A., in "Ion Exchange and Solvent Extraction" (J. A. Marinsky and Y. Marcus, Eds.), Vol. 11, p. 237. Dekker, New York, 1993; Miyajima, T., in "Ion Exchange and Solvent Extraction" (J. A. Marinsky and Y. Marcus, Eds.), Vol. 12, p. 275. Dekker, New York, 1995) has been applied in order to correct for the electrostatic effect on the equilibria inherent in the polyelectrolyte nature of the polyions. The potentiometric studies showed that the bidentate ligand complex, (CdA(2))(0), is the dominant species, but the formation of monodentate ligand complex, (CdA)(+), cannot be neglected at low degree of dissociation of the polyacid. The stability constants of the (CdA)(+) and (CdA(2))(0) complexes have been determined to be approximately 3.2 X 10 and 1.6 X 10(2), respectively. These values are in good agreement with the stability constants of 4.1 X 10 and 1.0 X 10(2) for Cd(acetate) and Cd(glutarate) complexes, where acetate and glutarate ions are the monomer analog ligands of polyacrylic acid. The Cd-113 NMR chemical shift change upon dissociation of the polyacid also indicates that the dominant complex is (CdA(2))(0), but (CdA)(+) becomes more appreciable with the decreasing degree of dissociation of the polyacid.