Photoreduction of carbon dioxide (CO2) to formate (HCO2-) can be achieved by UV-irradiation of the system involving phenazine (Phen) as a photosensitizer, a cobalt complex of cyclam (Co(III)L, L = cyclam = 1,4,8, 11-tetraazacyclotetradecane) as an electron mediator, and triethylamine (TEA) as an electron donor. Reduction products from the system are HCO2- and a small quantity of CO and H-2. The quantum yield is 0.07 for the formation of HCO2- at lambda = 313 nn. Preferential electron transfer from the photoformed radical anion of phenazine (Phen(.-)) to Co(III)L is confirmed by EPR analysis and the reaction between Phen(.-) and Co(III)L with a second-order rate constant, k = 4.3 x 10(9) M(-1) s(-1). The resulting Co(II)L reacts with phenazinyl radical,1 giving [CoL(H)](2+) by hydrogen transfer from phenazinyl radical. The effective insertion of a CO2 molecule into [CoL(H)](2+) yields HCO2- selectively.