Hydroxylapatite is generally used as a coating material on the metallic substrates, such as titanium alloys, stainless steels and CoCrMo alloys to take the advantageous of both excellent biocompatibility of hydroxylapatite and good mechanical properties of the metals at the same time. However, the bonding at the interface and the difference in the coefficient of thermal expansion between hydroxylapatite and the metals are crucial factor affecting the reliability of the coating layer. In this research the bonding of hydroxylapatite to 316L stainless steel and CoCrMo was studied. HA powder was sintered onto rods of the alloys by hot isostatic pressing. The interfaces between the HA and alloys showed interdiffusion profiles of all the HA and alloy elements, demonstrating interfacial bonding. There was also evidence of reaction of chromium from the alloys with apatite to form compounds. Chromium metal vapor deposited on HA showed evidence of the formation of CaCr2O4 in Rutherford backscattering (RBS). Thus HA bonds chemically to these alloys, and any interfacial cracking results from residual stresses caused by differences between coefficients of thermal expansion between the HA and the alloys. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.