Interaction between Ca 2+ ion and poly(styrene-alt-maleic acid) was studied by using a Ca 2+ ion sensitive electrode. The Ca 2+ activity had a peak at a degree of neutralization of 0.5 and decreased with increasing Ca(OH) 2 concentration beyond it when the polymer solution was neutralized with Ca(OH) 2. The decrease in the Ca 2+ activity was not observed when the polymer concentration was very low. The counter ion condensation theory did not hold for this solution except in the case of an extremely dilute solution. The additivity rule for Ca 2+ was confirmed for this solution. When the maleic acid copolymer was neutralized with both Ca(OH) 2 and KOH, the Ca 2+ activity had a peak at a degree of neutralization of 0.5 when neutralization with KOH was less than 0.3 and the Ca 2+ activity decreased more drastically than that neutralized with only Ca(OH) 2. The appearance of the peak of the Ca 2+ activity at a degree of neutralization of 0.5 was independent of the ratio of Ca 2+ concentration to polymer concentration or absolute Ca 2+ concentration, but depended on the degree of ionization, i.e., linear electric charge density on the polymer because of ionization of the carboxyl groups. Interpretations of the behavior of the Ca 2+ activity are discussed.