The reaction mechanisms of visible-light responsive Cu(II)-grafted Mo-doped SrTiO3 (Cu(II)/MoSTO) photocatalyst and related photocatalysts were analyzed by means of ESR spectroscopy for detecting trapped carriers at 77 K and chemiluminescence photometry for O-2(center dot-) and H2O2 in aqueous suspension. On the irradiation of visible light, the reduction of grafted Cu2+ ions was observed by ESR measurements for both Cu(II)/MoSTO and Cu(II)/STO(Cu(II)-grafted undoped SrTiO3) photocatalysts. One electron reduction product for O-2, i.e., O-2(center dot-) was detected in aqueous suspension by 442-nm irradiation for the Cu(II)-grafted samples. On the other hand, the amount of H2O2 detected as the two electron reduction product of O-2 was a very small scale, indicating possible further reduction of H2O2 into H2O. From the ESR observations of crystalline defects, it was concluded that the Mo doping to increase visible-light response also increase the electron-hole recombination to some extent, while the Cu(II) grafting functioned as a trap of excited electrons to decrease the recombination. (C) 2011 Elsevier B.V. All rights reserved.