Binding equilibria of a water-soluble chelating polymer, poly(acrylic acid), with divalent metal ions [Co(II), Ni(II), Cu(II), Zn(II), and Pb(II)] were measured by a batch ultrafiltration method in the pH range of 4-6. It was shown that the binding equilibrium curves were reasonably well represented by a complexation model accounting for the formation of three kinds of complexes [LM+, L2M, and L2M(HL)(2)]. 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 average coordination number was discussed on the basis of the complexation model proposed.