Abstract Chemical models describing the composition of the aqueous phase in equilibrium with hydroxy-apatite and fluorapatite solids have been established. The effect of atmospheric CO 2 contamination on the composition of these interfaces has been demonstrated. From these models, the amount of acid or base needed to maintain a given pH, the potential determining ions, and the points of zero net charge could be determined. Thus, the point of zero net charge for hydroxyapatite in water at equilibrium is at pH 8.65, which shifts to pH 7.05 if the system is allowed to contact atmospheric carbon dioxide. For fluorapatite the point of zero net charge is at pH 6.45 when air is present. Various aspects of apatite dissolution are also discussed.