The hydrolysis of a-tricalcium phosphate (alpha-TCP) in a simulated body fluid (SBF) at 37 degrees C was investigated. The hydration rate was found to be slower in SBF than that in deionized water. The concentration of ions in SBF was monitored by ICP. The hydrolysis product, which was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infra red, and X-ray photoelectron spectroscopy, was determined to be carbonate-containing, calcium-deficient hydroxyapatite (CO3-CDHAp) with Mg2+, Na+, and Cl- impurities similar to the biological apatite. An amorphous layer on the alpha-TCP surface was found to be the precursor of the apatite phase, which may either form crystalline apatite or may decompose back to alpha-TCP at a lower temperature.