Precipitated calcium orthophosphate, rapidly prepared in highly supersaturated solutions at pH 9-11, is an amorphous tricalcium orthophosphate (atomic ratio Ca/P = 3/2) of formula Ca9(PO4)6, nH2O. Kept wet at room temperature, this phosphate is hydrolyzed according to the reaction PO4(3-) + H2O leads to HPO4(2-) + OH-; a tricalcium orthophosphate series is then formed, its general formula is Ca9 (HPO4)x (PO4)6-x (OH)x, O less than or equal to x less than or equal to 1. Moreover, the amorphous phosphate is converted into apatitic phosphate at half hydrolysis (x approximately equal to 0.5). The hydrolysis occurs simultaneously with an endothermic effect [delta H = 12.0 +/- 2.0 kJ/mol Ca9(PO4)6], and the conversion with an exothermic effect [delta H = -5.9 +/- 1.0 kJ/mol Ca9(PO4)6].