Binding equilibria of a water-soluble chelating polymer, poly(alpha-acethylaminoacrylic acid), with divalent metal ions (Co(II), Ni(II), Cu(II), Zn(II), Pb(II)) were measured by a batch ultrafiltration method in the pH range 4-6. It was shown that the binding equilibrium curves were reasonably well represented by the complexation model accounting for the formation of three kinds of complexes (LM+, L2M, and L2M(HL)(2)) proposed previously. The equilibrium and successive stability constants were determined by fitting the binding curves calculated from the model to the experimental ones at every solution pH. The variations of the average coordination number depending on solution pH and metal ion concentration were well explained on the basis of the relative concentration of each complex to total polymer.