The reduction of CO2 and H+ by Ni(cyclam)(+) in aqueous solution was found to proceed by the reduction of Ni(cyclam)(CO2)(+) or Ni(cyclam)(H)(2+) by Ni(cyclam)(+). At ambient temperature the rate constants for the bimolecular reactions are 1.6 x 10(8) and 7.2 x 10(7) M-1 s(-1), respectively. Continuous gamma-radiolysis studies have demonstrated nearly quantitative formation of CO from two Ni(cyclam)(+) at pH 5.2 under 1 atm of CO2 and in the presence of formate. Under the same conditions, but at low CO2 concentration, the reduction of protons was found to compete with CO2 reduction.