The formation of radical species on stoichiometric hydroxyapatite (HAp) by UV irradiation and the photocatalytic behavior of HAp were studied. The radical species formed on HAp after heat treatment at 200degreesC (HAp200) by UV irradiation was assigned to be O-2(.-) species from the results of electron spin resonance (ESR) studies, Electron transfer must occur from vacancy formed in apatitic structure to oxygen in atmospheric air on HAp. It was recognized that the amounts of O-2(.-) species on HAp increased by UV irradiation. The conversion of methyl mercaptane (MM) using HAp200 under UV irradiation for 60 min was 96%. That is, the photocatalytic decomposition of MM on HAp200 proceeded effectively. It was suggested that the photocatalytic activity of HAp was due to the production of sufficient amounts of O-2(.-) species under UV irradiation. (C) 2002 Elsevier Science B.V. All rights reserved.