The quenching behaviors of triplet benzophenone by some amines in solution have been directly investigated using the nanosecond laser photolysis technique. It has been confirmed in systems of benzophenone and tertiary aromatic amines, such as N,N-diethylaniline and N,N-dimethyl-p-toluidine, that the ionic photodissociation to benzophenone anion and amine cation takes place completely in acetonitrile, while the photoreduction of benzophenone occurs efficiently in benzene. These processes were found to compete with each other depending largely upon solvent polarity. The quenching of the triplet benzophenone by primary and secondary aromatic amines as well as aliphatic amines leads to the formation of a ketyl radical in all solvents used. The marked distinction observed in the quenching processes of triplet benzophenone by various amines has been explained by the difference in ionization potential and the change of molecular structure of the amine in addition to the solvent effect. The solvent effect was discussed on the basis of free-energy calculations regarding the formation of ion pairs and free ions involved in the quenching path. In the case of the benzophenone-tertiary amine system it is concluded that an electron transfer followed by proton transfer results in: rapid hydrogen abstraction. The relaxation time for solvent orientation may be a useful guide to the present conclusion. In the case of primary and secondary amines the importance of a hydrogen atom bonded to the N atom of the donor was suggested.