A very active type of hydroxyapatite (HAp-2) for photocatalytic decomposition of dimethyl sulfide (DMS) under UV irradiation and its structural characteristics were studied. The conversion of DMS using HAp-2 was 98-100% at the space velocity of 340 h(-1) under UV irradiation, and the activity of HAp-2 was higher than that of another type of HAp (HAp-1). The photocatalytic decomposition of DMS proceeded oxidatively and effectively on HAp-2 under UV irradiation. It was recognized that DMS converted stoichiometrically to CO2 on HAp-2. From the X-ray diffraction (XRD) studies, the HAp-1 crystals had a lower crystallinity than crystals of HAp-2 toward the apatite a-axis, but a similar crystallinity toward the c-axis. Then, it was concluded that the photocatalytic activity of HAp must be due to the crystallinity toward the a-axis of HAp crystal. (C) 2003 Elsevier B.V. All rights reserved.