Radiolysis of aqueous solutions containing CO2 and Cu(II) ions under reducing conditions leads first to reduction of the Cu(II) and then to formation of CO. Experiments under various conditions show that although Cu(II) was often reduced to colloidal Cu(0) particles, formation of CO requires the presence of Cu(I). It also requires that CO2 be first reduced to the (CO2-)-C-. radical. The (CO2-)-C-. radical was produced radiolytically by reaction of CO2 with e(aq)(-) or by reaction of formate ions with H-. and (OH)-O-. radicals and photochemically by reaction of formate with the acetone triplet. The (CO2-)-C-. radicals are reduced to CO via addition to Cu(I) and subsequent reaction of the product with another Cu(I). The first reaction produces CuCO2, which undergoes protonation at pH < 4. The reaction of the neutral CuCO2 with Cu(I) leads to reduction of the copper to form Cu2+ and subsequently Cu(0) particles. However, the reaction of the protonated form, CuCO2H+, with Cu(I) leads to oxidation of the copper and formation of CO in the form of the CuCO+ complex. After most of the copper is converted into CuCO+, subsequent reactions involve this species instead of Cu+ and lead to further production of CO. From pulse radiolysis measurements, the rate constants for the reactions of the (CO2-)-C-. radicals with Cu+ and CuCO+ were found to be approximate to 1 x 10(9) and (1.5 +/- 0.4) x 10(8) L mol(-1) s(-1), respectively. The protonated adduct formed by the latter reaction at pH 3.4, Cu(CO)CO2H+, reacts with CuCO+ with a rate constant approximate to 5 x 10(5) L mol(-1) s(-1) to produce more CO. It also undergoes first-order decomposition and second-order decay reactions.