Titanium(IV) oxide (TiO2) particles were modified with several kinds of transition-metal ions (iron(III), copper(II), nickel(II) and chromium(III) ions). Photocatalytic activity for acetaldehyde decomposition over metal-ion-modified TiO2 showed higher photocatalytic activity than bare TiO2 under ultraviolet (UV) as well as visible-light irradiation. The mechanism of photocatalytic reaction over metal-ion-modified TiO2 was investigated by double-beam photoacoustic (DB-PA) spectroscopy, which enables observation of electron migration in the TiO2 particles. DB-PA measurements suggested that metal ions on TiO2 surface acted differently depending on the wavelength of photoirradiation, i.e., as electron acceptors under UV irradiation and as electron injectors under visible-light irradiation. (C) 2008 Elsevier B.V. All rights reserved.