Isomorphic substitution of (Si, S) for phosphorus in apatite (Ca5(PO)3F) was investigated in the solid solution series from P-apatite to (Si, S)-apatite utilizing compositions of Ca5(PO4)3−x (SiO4)x/2(SO4)x/2F with x=0, 0.75, 1.5, 2, 2.25, 3. Study of the solubility characteristics of these apatites showed an increased weight loss in water with progression from the P-apatite end member to the (Si, S)-apatite end member. Experimentally determined pKsp decreased from 60.62 for the fluorapatite (Ca5(PO4)3F) to 31.65 for fluorellestadite (Ca5(SiO4)1.5(SO4)1.5F). The free energy of formation calculated from the solubility product became less negative from −6755 kJ mol −1 for fluorapatite to −5760 kJ mol −1 for fluorellestadite.