Photoinduced electron-transfer processes from the excited triplet states of zinc tetraphenylporphyrin ((ZnTPP)-Zn-3*) or zinc tetra-tert-butylphthalocyanine ((ZnTBPc)-Zn-3*) to oxo-acetato-bridged triruthenium clusters [Ru-3(mu(3)-O)(mu-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 ([Ru-3(mu(3)-O)(mu-CH3CO2)(6)(L)(3)](0)) were observed with the concomitant decays of (ZnTPP)-Zn-3* or (ZnTBPc)-Zn-3*. The evaluated rate constants (k(ET)) and quantum yields ((Phi(ET)) 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 [Ru-3(mu(3)-O)(mu-CH3CO2)(6)(L)(3)](+). The Phi(ET) values for (ZnTPP)-Zn-3* were lower than those for (ZnTBPc)-Zn-3*, suggesting the presence of competitive processes such as energy transfer process from (ZnTPP)-Zn-3* to the triplet states of [Ru-3(mu(3)-O)(mu-CH3COO)(6)(L)(3)](+). For the back electron-transfer process, second-order kinetics indicates that the radical cations of ZnTPP or ZnTBPc and [Ru-3(mu(3)-O)(mu-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.