Dissociation behavior of poly(oc-hydroxy acrylic acid) (PHA) was investigated by potentiometric titration in the presence of NaCl and/or divalent metal chlorides. It was found that pH values of PKA aqueous solutions increased with time when the degree of dissociation, a, is high (alpha greater than or equal to 0.5 for NaCl system) and decreased in the lower alpha region (alpha < 0.5 for NaCl). The increase of pH was attributed to lactone ring formation that occurs between a carboxyl group and a neighbor hydroxyl group upon protonation to the former, and the decrease of pH to an induction effect by lactone ring to -COOH group. The pH-increasing process was analyzed by assuming it being of a first order to obtain a time constant. On the basis of time constant thus estimated and pKa values for divalent counterion systems, a correlation between counterion binding and lactone ring formation was discussed.