Interaction of Ca(2+) ions with poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMA) was studied using a Ca(2+) ion sensitive electrode. When the PAA solution was neutralized with Ca(OH)(2), the Ca(2+) activity had a maximum around the degree of neutralization 0.5 and decreased with further increase of Ca(OH)(2), being similar to the behavior of the Ca(2+) activity in a maleic acid copolymer solution as reported previously. When the polymer concentration was as low as 0.1 mM, this peak in the Ca(2+) activity was not observed and the counterion condensation theory held. The decrease of the Ca(2+) activity in PAA solution at the degree of neutralization unity was depresseed by the presence of several millimolar KCl. The Ca(2+) activity in the PAA solution at the low degree of neutralization was increased by the presence of dilute KCl and decreased by the presence of concentrated KCl. The decrease of the Ca(2+) activity in PMA solution was observed also at the degree of neutralization unity, but its extent was small compared with that of the PAA solution and the maximum of the Ca(2+) activity was shifted to the degree of neutralization 0.75. The effects of KCl on the Ca(2+) activity in the PMA solution were almost the same as those in the PAA solution. Interpretations of the behavior of the Ca(2+) activity were discussed.