The activity product of tetracalcium phosphate (TTCP, Ca4(PO4)20), was determined at 37 ~ and the hydrolysis of TTCP was investigated in 0.01-0.1 mol 1-1 H3PO4 and KH2PO4 solutions by means of calcium and phosphorus analyses, X-ray diffraction and infrared analysis. The activity product, defined as Ksp=(Ca2+) 4 (PO]-) 2 (OH-) 2, was 37.36 as pKsp, which was smaller than that previously reported (42.4). TTCP easily hydrolysed to form calciumdeficient apatite (Ca-def OHAp, Cas-x(HPO4)x(PO4)3-x(OH)l-x), or dicalcium phosphate dihydrate (DCPD, CaHPO42H20), depending on the initial phosphate concentration. With 0.1 mol I -I H3PO4, TTCP hydrolysed to form DCPD within several minutes. In 0.025 mol 1-1 H3PO4 and 0.1 moll -1 KH2PO4, TTCP hydrolysed to form Ca-def OHAp through DCPD. In the latter solution, a small amount of octacalcium phosphate (OCP, Cas(H2PO4)2(PO4)45H20), was detected as an intermediate product. In 0.025 mol 1-1 KH2PO4, TTCP hydrolysed directly to form Ca-def OHAp. In 0.01 moll -1 HaPO4, hydrolysis of TTCP was not completed, although Ca-def OHAp was only a product. Thus the final product and the degree of hydrolysis depended on the pH and the overall Ca/P ratio in the reaction system. The rate of Ca-def OHAp formation seemed to be controlled by the dissolution rate of TTCP rather than the crystallization rate of the OHAp.