Ferrous ions are found to catalyze the photochemical reduction of CO2 to CO in acetonitrile (ACN) solutions containing triethylamine (TEA) or triethanolamine (TEOA) as reductive quenchers and p-terphenyl (TP) as a photosensitizer. TP is photoreduced to form the radical anion, TP.-, which reduces Fe(II) ions. The rate constant for the reduction of Fe(II) by TP.- was determined by pulse radiolysis in ACN/TEOA solutions and found to be (1.2 +/- 0.4) x 10(8) L mol(-1) s(-1). The Fe(I) ions produced are expected to react with CO2 to form an adduct that is equivalent to the adduct known to be formed upon reaction of CO2.- with Fe(II) complexes. Subsequent reduction of the Fe-CO2 adduct leads to formation of CO. Fe(I) can also undergo protonation and form H-2. After extensive irradiation, photochemical production of CO stops, probably due to competition between CO and CO2 for the Fe(I) binding sites. Addition of CO to the solution enhances the photochemical production of H-2.