Hydrolysis of tetracalcium phosphate (TTCP) that forms calcium hydroxyapatite (HA) is a process important for the setting of calcium phosphate cements used as bone graft materials. Objective: The present study investigated the influence of solution pH and calcium (Ca) and phosphate (P) concentrations on TTCP hydrolysis and the composition of the HA products. Methods: 1 g of TTCP (particle size 13-20 mm) was hydrolyzed in 100 mL of a 0.15 mol/L KNO3 solution containing 0, 0.05, or 0.25 mol/L of Ca(NO3)2 or K2HPO4 with pH adjusted to 7, 8, or 10. The solution composition was kept constant during the hydrolysis (37 °C, constant stirring, N2 atmosphere) by addition of H3PO4 triggered by a combination pH electrode. The HA products were analyzed chemically and by x-ray diffraction. Results: The composition of the HA products varied significantly with solution composition. At pH = 7, the Ca/P ratio (expressed as mean ± s.d.; n = 2) of HA formed in solution that initially did not contain Ca or P was 1.50 ± 0.02. The Ca/P of HA formed in solutions containing 0.05 mol/L Ca, 0.25 mol/L Ca, 0.05 mol/L P, and 0.25 mol/L P were 1.57 ± 0.01, 1.59 ± 0.02, 1.42 ± 0.01, and 1.32 ± 0.07, respectively. At pH = 8, the Ca/P increased compared to the corresponding values at pH = 7. The same trend continued at pH = 10, but only a portion of TTCP was hydrolyzed. At given pH, the initial rate of TTCP hydrolysis was higher in K2HPO4 than in Ca(NO3)2 containing solutions. Conclusions: An increase in pH or Ca concentration retarded the TTCP hydrolysis and increased the Ca/P ratio of the HA products. An increase in P concentration promoted hydrolysis and reduced Ca/P ratio of the HA. Supported by NIDCR grant DE11789, ADAHF and NIST.