The photoeffects of trapped electrons (Ti3+) in partially reduced TiO2 nanoparticles were studied by in situ ESR spectroscopy. For the pure phase samples, upon illumination with an energy lower than the band gap, the Ti3+ spectra decreased in intensity; however, the initial ESR signal was restored within 30 min in the dark at 77 K. For the TiO2 (P-25) particles, the ESR line shape changed significantly with being constant in the integrated intensity of the signal after the restoration. It was found that the trapped electrons can transfer from the anatase phase to the rutile phase in TiO2 (P-25) by photoexcitation. (c) 2006 Elsevier B.V. All rights reserved.