The transformations brushite → fluoroapatite and monetite → fluoroapatite have been investigated at 25 °C in aqueous acidic KF solutions, ca. 10–1 M in F–. The transformations involve a first-order uptake of F– by the secondary orthophosphates, followed by a solid-state rearrangement to fluoroapatite with the second-order release of H2PO4–. The ΔGf° for fluoroapatite has been calculated from the brushite → apatite transformation as –1524·1 kcal mol–1, and confirmed by calculation of the ΔGf° value for monetite as –401·1 kcal mol–1. From these results, the ΔGf° values of hydroxyapatite, Ca5OH(PO4)2, Ca3(PO4)2, and Ca(H2PO4)2, and Ca(H2PO4)2,H2O were calculated as –1494·2, –925·1, –677·8, and –729·5 kcal mol–1. The possibility of K substitution for Ca in fluoroapatite is discussed.