The one-electron redox processes of several compounds during polyoxometalate (POM)-mediated photocatalytic reactions of TiO2 were investigated using the two-color two-laser flash-photolysis technique. The efficiency of the one-electron oxidation of aromatic sulfides by the trapped hole (htr+) or the surface-bound OH radical (OHs.) is found to be significantly enhanced due to electron transfer from the conduction band (CB) of TiO2 to the POM. The efficiency of the electron transfer from the CB of TiO2 to the POM decreases in the order H2W12O406− < SiW12O404− < PW12O403−, that is, it depends on the reduction potential (Ered) of the POMs. Electron injection from PW12O404− in the excited state (PW12O404−*) to the CB of TiO2 was clearly observed using the two-color two-laser flash-photolysis technique. Storage of electrons in the TiO2/PW12O403−/methyl viologen (MV2+) ternary system was also achieved upon two-color two-laser irradiation.