The formation of metal complexes between poly (acrylic acid) and divalent, trivalent and tetravalent metal ions was studied. For divalent metal ions, the formation constants determined according to the method of Gregor et al. strongly decrease with increasing ligand concentration in the range 25-280 mM but are nearly insensitive to the ligand to metal concentration ratio. The metal ions were divided in four groups depending on their effect on the pH decrease of the solution. These groups differ from one another by i) the speed of water exchange or ii) by the speed of ligand exchange in the coordination sphere of the metal. The increase of the ligand concentration or of the ligand to metal ratio always led to an increase of the exchange speed of both water and carboxyl groups.