The portion of the stable phase diagram for the ternary system CaO‐P2O5‐H2O involving calcium phosphate phases more basic than Ca(H2PO4)2has been constructed. Construction involves plotting concentrations as their fifth roots to allow phases having very different solubilities to be represented on a single diagram. Hydroxyapatite is shown to be congruently soluble. The stability region for hydroxyapatite includes compositions having Ca/P ratios extending between 1.5 and 1.67. Calcium‐deficient hydroxyapatite forms a stable invariant point with CaHPO4and a metastable invariant point with CaHPO4. 2H2O. Inspection of the diagram indicates that the compounds CaHPO4and CaPHO4.2H2O dissolve incongruently. Incongruent dissolution of either CaHPO4or CaHPO4.2H2O results in the formation of hydroxyapatite during the dissolution process. Such incongruency between these compounds and hydroxyapatite influences their microstructural relation