Pulse radiolysis and laser flash photolysis have been used to generate and characterize Ni(cyclam)(+), where cyclam is 1,4,8,11-tetraazacyclotetradecane, in aqueous solution. Reduction of Ni(cyclam)(2+) was carried out at ambient temperature (22 +/- 2 degrees C) by solvated electrons, e(aq)(-), hydrogen atoms, H-., and carbon dioxide radical anions, CO2.-. Reduction by e(aq)(-) occurs with a rate constant of (4.1 +/- 0.2) x 10(10) M(-1) s(-1) (I less than or equal to 2 x 10(-4) M). Reduction by CO2.- and H-. proceeds via an inner-sphere mechanism to yield [Ni(cyclam)(CO2)](+) and [Ni(cyclam)(H)](2+), with rate constants of(6.7 +/- 0.2) x 10(9) and (5 +/- 2) x 10(9) M(-1) s(-1), respectively. Decomposition of the adducts, to yield Ni(cyclam)(+) and Co-2 or H+, proceeds with rate constants of (2.0 +/- 0.2) x 10(6) and (5.3 +/- 0.7) x 10(5) s(-1), respectively. Carbon dioxide and proton dissociation constants for [Ni(cyclam)(Co-2)](+) and [Ni(cyclam)(H)](2+) were found to be (6.2 +/- 0.3) x 10(-2) and (1.6 +/- 0.4) x 10(-2) M, respectively. The rate constants for the addition of CO2 and H+ to Ni(cyclam)(+) were calculated to be (3.2 +/- 0.4) x 10(7) and (3 +/- 1) x 10(7) M(-1) s(-1), respectively. The selective reduction of CO2 by Ni(cyclam)(+) in the presence of protons at pH 4 is rationalized.