p-Terphenyl (TP) sensitizes photocatalytic reduction of water to H-2 under UV irradiation of homogeneous aqueous acetonitrile solution in the presence of triethylamine (TEA) as an electron donor and Ru3+ as a precursor of cocatalyst, Ru metal colloid. The addition of quaternary onium salts enhances the H-2 evolution, where the onium salts with longer alkyl groups become more effective. Dynamics studies of TP photosensitization reveal that the presence of the salts contributes to stabilization of the radical anion of TP (TP.-) formed through reductive quenching of the singlet state of TP ((TP)-T-1*) and the triplet state of TP ((TP)-T-3*) by TEA. The TP photosensitization accompanies competitive photo-Birch reduction via TP-, but the presence of onium salts enhances the lifetime of TP.- through the specific interaction, leading to the effective TP-photosensitized H-2 evolution.