The dissolution of octacalcium phosphate (OCP), a possible important precursor phase in the in vivo mineralization of hard tissue, has been studied at 37 degrees C over a range of undersaturations using a constant composition method (CC). The rate decreases markedly with time despite the sustained driving force, and eventually it approaches zero even though crystals remain in the undersaturated solutions. These OCP crystals will not dissolve when exposed to calcium phosphate solutions of the same undersaturations. However, the dissolution reaction resumes if the undersaturations are changed. Furthermore, at low undersaturation, the CC OCP dissolution curves show characteristic stepwise profiles. These interesting unpredicted phenomena persuade us to examine in more detail current crystal growth/dissolution models.