The suspension pH at a given concentration of hydroxyapatite (HAP) decreased with the concentration of calcium chondroitin-6-sulfate (CaChs), whereas it increased with the concentration of sodium chondroitin-6-sulfate (Na2Chs). The former effect is due to the increase in the concentration of H+ by ion exchange between H+ on the surface of HAP and Ca2+ of CaChs, and the latter is due to the protonation of phosphate ion (Pi) released from the surface of HAP. The absorbed amount of chondroitin-6-sulfate anion (Chs) by HAP was higher with CaChs than with Na2Chs over the concentration range examined. The equilibrium concentration of Pi decreased with increasing concentration of added CaChs because the concentration of free Ca2+, which dissociates from CaChs, regulates the free concentration of Pi through the restriction of the solubility product of HAP (Ksp). In contrast, that in the presence of Na2Chs increased with increasing concentration of added Na2Chs owing to the anion exchange between Chs and Pi of the HAP surface. The total concentration of Ca2+, which was released from HAP into the solution phase, increased after passing through a minimum with increasing concentration of added Na2Chs. That is, the concentration of Ca2+ free from Chs decreased with an increase in the concentration of released Pi owing to the restriction of the solubility product, whereas that of Ca2+ bound by Chs increased with increasing concentration of added Na2Chs through the ion exchange of Na+ with Ca2+. It was confirmed by the dialysis method that the value of Ksp was almost constant around 10, although HAP dissolves incongruently in the presence of Na2Chs.