Photoinduced electron-transfer processes from the excited triplet states of zinc tetraphenylporphyrin (3ZnTPP*) or zinc tetra-tert-butylphthalocyanine (3ZnTBPc*) to oxo-acetato-bridged triruthenium clusters [Ru3(μ3-O) (μ-CH3CO2)6-(L)3]+ have been confirmed by nanosecond laser flash photolysis in the visible and near-IR regions. The rise of the transient absorption spectra of the radical cations of ZnTPP and ZnTBPc and the reduced form of the oxo-acetato-bridged triruthenium cluster ([Ru3(μ3-O)(μ-CH 3CO2)6(L)3]0) were observed with the concomitant decays of 3ZnTPP* or 3ZnTBPc*. The evaluated rate constants (kET) and quantum yields (ΦET) for electron-transfer were increased with the order of electron-withdrawing ability of the ligands (L) coordinated to the Ru atoms, 4-cyanopyridine > triphenylphosphine > pyridine > 4-(dimethylamino)pyridine, which is the order of promoting the electron-accepting ability of [Ru3(μ3-O)(μ-CH 3CO2)6(L)3]+. The ΦET values for 3ZnTPP* were lower than those for 3ZnTBPc*, suggesting the presence of competitive processes such as energy transfer process from 3ZnTPP* to the triplet states of [Ru3(μ3-O)(μ-CH3COO) 6(L)3]+. For the back electron-transfer process, second-order kinetics indicates that the radical cations of ZnTPP or ZnTBPc and [Ru3(μ3-O)(μ-CH3COO) 6(L)3]0 return to the original system after solvation in polar solvents at a diffusion controlled limit without side reactions, providing reversible photosensitizing intermolecular electron-transfer systems.