The addition of CO to Ni(cyclam)(+) in homogeneous aqueous solution has been studied by pulse radiolysis and laser flash photolysis. The reaction was found to proceed by a sequential, two-step mechanism. The CO addition rate constant was found to be (2.0 +/- 0.2) x 10(9) M(-1) s(-1) at ambient temperature. Addition is followed by a slower, first-order reaction with a rate constant of 1.8 +/- 0.2 s(-1) at ambient temperature. A variable temperature study of the latter reaction yielded Delta H-not equal = 12.7 +/- 0.1 kcal mol(-1) and Delta S-not equal = -14.4 +/- 0.1 cal K-1 mol(-1). The mechanism of the reaction sequence and the possibility that the slow, first-order process represents an isomerization of the macrocyclic ligand are discussed.