Divalent counterion binding has been investigated for Na poly(oc-hydroxy acrylate) (PHANa) in the presence of MgCl2, CaCl2 or BaCl2 by measuring the reduced viscosity, apparent turbidity and divalent counterion activity of the pertinent aqueous solutions. It was found that the viscosity decreases in the order, Mg2+ much greater than Ba2+ greater than or equal to Ca2+, while the turbidity at the devalent counterion ratio to the polymer, C-2/C-p similar to 0.5, was in the order, Mg2+ > Ca2+ greater than or equal to Ba2+ The former order proved to be consistent with that from the activity measurement. The apparent contradiction suggested by the result of the turbidity measurement has been interpreted in terms of 'hydration force'. Reference experiments with Na poly(acrylate) (PAANa) demonstrated that PHANa is able to bind Ca2+ over Mg2+ much more effectively than PAANa, probably through coordination of the alpha-hdyroxyl group to the former counterion. (C) 1997 Elsevier Science Ltd.