Surface changes of stoichiometric hydroxyapatite (HAp) by UV irradiation and radical formation were studied to make clear the factors of the photo-induced catalytic behavior. From the FTIR and XPS studies it was shown that the surface of HAp after heat treatment at 200 degreesC (HAp 200) became electroconductive by UV irradiation, and the UV irradiation caused the changes of the surface PO4 group, probably the formation of oxygen vacancy. From the result of the ESR spectrum obtained by subtracting the spectrum of HAp 200 before UV irradiation from that after UV irradiation, the signal at g = 2.0030 was assigned to the electron trapped on vacancy. It was concluded that the appearance of the electron trapped on vacancy, in HAp 200 under UV irradiation, must cause the formation of the very active O-2(.-) species. (C) 2003 Elsevier B.V. All rights reserved.