THE free enthalpy of formation of solid solutions of hydroxyapatite (OHA) and fluorapatite (FA) at 25° C has been determined1. I have used these thermodynamical data to calculate (Driessens, unpublished) the solubility of the solid solutions in aqueous solutions at certain pH values. For this purpose, the following relations were derived: where subscript (s) means that the preceding molecular unit is part of the solid solution; subscript (1) means that the preceding ion is dissolved in the aqueous solution; K1OHA and K1FA are the apparent solubility products of the components OHA and FA over the solid solution respectively: they depend on x; KOHA and KFA are the solubility products of pure OHA and pure FA respectively: they are independent of x; aOHA and aFA are the thermodynamical activities of FA and OHA, which are sufficient to account for the dependence on x.