Data relating to the ionic activities of supernatant solutions arising from the fluoride-induced transformations of monetite, CaHPO4, and brushite, CaHPO4,2H2O, were used to calculate ΔGf° of Ca2F(PO4), Ca3(PO4)2, Ca(H2PO4)2, and Ca(H2PO4)2,H2O as –600·3, –926·1, –675·2, and –731·8 kcal mol–1 respectively. The stabilities of the various calcium phosphates in the system CaO–P2O5–H2O have been investigated under aqueous conditions relative to the pH. The relationship at 25°C is of the form Ca(H2PO4)2,H2O ⇌ CaHPO4,2H2O (pH 2·3); CaHPO4,2H2O ⇌ Ca8H2(PO4)6,5H2O (pH 4·85); Ca8H2(PO4)6,5H2O ⇌ Ca5OH(PO4)3(pH 5·95); Ca5OH-(PO4)3⇌ Ca4O(PO4)2(pH 6·55). The Ca8H2(PO4)6,5H2O ⇌ Ca5OH(PO4)3 and Ca5OH(PO4)3⇌ Ca4O-(PO4)2 transformations are dependent on the activity of Ca2+, and show increase in pH with decrease in the activity. Preliminary results have also been obtained of the relationship existing under aqueous conditions at 25°C in the system CaO–P2O5–CaF2–H2O, which may be summarised as Ca4O(PO4)2⇌ Ca2FPO4(pH 8·05); Ca2FPO4⇌ Ca5F(PO4)3, (pH 6·63); Ca5F(PO4)3⇌ CaF2(pH 2·10).