The photocatalytic activity of Ti(IV)-doped calcium hydroxyapatite (TiHap) particles with atomic ratio of Ti/(Ti + Ca) [X-Ti] from 0 to 0.20 prepared by a coprecipitation method, called as original particles, were examined by in situ Fourier-Transform IR (FTIR) measurement. The heat treated TiHap particles were also used after treated original particles at 650 degrees C for 1 h, called as heat treated particles. All the original particles were made up by rod-like particles with irregular size, while the heat treated particles were rounded by sintering and their sizes were enlarged in each X-Ti system. The TiHap particles were cation deficient particles with (Ca + Ti)/P atomic ratio ranging between 1.54 and 1.64. The X-Ti values of TiHap particles assayed showed approximately the dosage of Ti(IV) ions in each system. All the TiHap particles employed exhibited UV absorption below ca. 370 nm, though the UV absorption characteristics did not vary by the heat treatment. However, the pure calcium hydroxyapatite with X-Ti = 0 had no UV absorption. The IR spectra of the particles with X-Ti >= 0.05 exhibited a new large peak at 3428 cm(-1) close to the peak at 3570 cm(-1) of OH- ions together with the bands at 3654-3682 or 3673 (surface P-OH groups) and 3737 cm(-1) (surface Ti-OH groups). The deuteration method revealed that the new band at 3428 cm(-1) could be assigned to the dislocated OH- ions by substituting Ti(IV) ions. The expansion of crystal structure along with c direction by the heat treatment was identified by the unit-cell dimension obtained from the XRD patterns. It was concluded therefore that the produced dislocated OH- ions by the heat treatment induce the photocatalytic activity of TiHap particles. The strong photocatalytic activity of TiHap particles was attributed to the dislocated OH- ions because they can produce *OH radicals in the particles without water molecules by UV irradiation. Furthermore, FTIR and XRD measurements elucidated that the optimum temperature for inducing photocatalytic activity of TiHap is ca. 600 degrees C. (c) 2012 Elsevier B.V. All rights reserved.