Formation of hydroxyapatite (HA) by hydrolysis of dicalcium phosphate anhydrous (DCPA) is a process important for the setting of calcium phosphate cements used as bone graft materials. Objective: The present study investigated the influence of DCPA particle size, solution pH and excess of calcium on the duration of DCPA hydrolysis and the Ca/P ratio of the HA products. Methods: 1 g of DCPA (median particle size 1.4±0.5 µm or 19±1.0 µm) was hydrolyzed in 100 mL of a 0.15 mol/L KNO3 solution containing 0 or 0.25 mol/L of Ca(NO3)2 at pH 7 or 10. The pH of solution was kept constant during the hydrolysis (37°C, stirring speed 400 rpm, N2 atmosphere) by addition of Ca(OH)2-titrant-solution triggered by a combination pH electrode. The rate of titration indicated the rate of hydrolysis. The Ca/P ratios of HA products were calculated from the amounts of initial DCPA and Ca(OH)2-titrant-solution. The products were analyzed chemically and by x-ray diffraction. Results: The duration of hydrolysis (t) and the Ca/P ratio of HA product (both expressed as mean ± s.d.; n = 2) in solution that initially did not contain Ca were (a) for small DCPA (1.4µm): t=75±3 h, Ca/P=1.44±0.01 at pH=7 and t=15.5±0.8 h, Ca/P=1.66±0.03 at pH=10, and (b) for coarse DCPA (19µm): t=252±10 h, Ca/P=1.44±0.01 at pH=7 and t=55±3 h, Ca/P=1.56±0.02 at pH=10. For small DCPA hydrolyzed in solutions containing 0.25 mol/L Ca the corresponding values were: t=5.2±0.4 h, Ca/P=1.63±0.03 at pH=7, and t=2.2±0.2 h, Ca/P=1.65±0.02 at pH=10. Conclusions: An increase in DCPA particle size increased the duration of hydrolysis reaction. An increase in pH promoted DCPA hydrolysis and decreased the Ca-deficiency of the HA products. At a given pH, Ca(NO3)2 solution increased the rate of DCPA hydrolysis. Supported by NIDCR grant DE11789, ADAF and NIST.