We previously reported that photoformed O-3(-) species on highly dispersed titanium oxide species are the active oxygen species for the epoxidation of propene. In the present study, it was found that this epoxidation system could be extended to other light alkenes such as ethene and butene, and the electronic structure of the O-3(-) species and reaction mechanism of the insertion of atomic oxygen into the C=C bond in olefin were revealed. An isotope-labeled reaction test with O-18(2) and ESR measurement with O-17(2) clarified the following mechanism: the O-3(-) species is generated by the reaction between an O-2 molecule and a photoformed hole center on lattice oxygen (O-L(-)), and the photoformed hole in the O-3(-) moves from the lattice oxygen (O-L(-)) to the O-2 moiety derived from molecular oxygen. The O-3(-) species can (electronic neutral) and adsorbed O-L(2-). This electron-deficient oxygen moiety derived from be also described as a surface complex of molecular oxygen could preferably react with the electron rich part of alkene, the C=C bond, to produce epoxide. (c) 2005 Elsevier Inc. All rights reserved.