Photoreduction of methyl viologen (MV² ) by eosin-Y (EY²⁻) in the presence of triethanolamine (TEOA) has been investigated in water–methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV^· ) was observed in the presence of lower concentrations of EY²⁻ and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY²⁻ [³(EY²⁻)^*] to MV² in the rate constants of ca 2.0 × 10¹⁰ M⁻¹ s⁻¹. The rates and efficiencies of production of MV^· were found to be dependent on solvent compositions and concentrations of MV² ionic salt and TEOA. The back electron transfer reaction from MV^· to EY^·− was retarded in the presence of TEOA, which supports that EY²⁻ is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV^· from EY^·3− which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of ³(EY²⁻)^* for the MV^· formation have been confirmed.