The activity product of tetracalcium phosphate (PTCP, Ca-4(PO4)(2)O), was determined at 37 degrees C, and the hydrolysis of TTCP was investigated in 0.01-0.1 mot l(-1) H3PO4 and KH2PO4 solutions by means of calcium and phosphorus analyses, X-ray diffraction and infrared analysis. The activity product, defined as K-sp=(Ca2+)(4) (PO43-)(2) (OH-)(2), was 37.36 as PKsp, which was smaller than that previously reported (42.4). TTCP easily hydrolysed to form calcium-deficient apatite (Ca-def OHAp, Ca-5-x(HPO4)(x)(PO4)(3-x)(OH)(1-x)), or dicalcium phosphate dihydrate (DCPD, CaHPO(4)2H(2)O), depending on the initial phosphate concentration. With 0.1 mol l(-1) H3PO4, TTCP hydrolysed to form DCPD within several minutes. In 0.025 mol l(-1) H3PO4 and 0.1 mol l(-1) KH2PO4, TTCP hydrolysed to form Ca-def OHAp through DCPD. In the latter solution, a small amount of octacalcium phosphate (OCP, Ca-8(H2PO4)(2)(PO4)(4)5H(2)O), was detected as an intermediate product. In 0.025 mol l(-1) KH2PO4, TTCP hydrolysed directly to form Ca-def OHAp. In 0.01 mol l(-1) H3PO4, 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.