Cobalt macrocycles mediate electron transfer in the photoreduction of CO2 with p-terphenyl as a photosensitizer and a tertiary amine as a sacrificial electron donor in a 5:1 acetonitrile/methanol mixture. The mechanism and kinetics of this system have been studied by continuous and flash photolysis techniques. Transient spectra provide evidence for the sequential formation of the p-terphenyl radical anion, the Co(I)L(+) complex, the [Co(I)L-CO2](+) complex, and the [S-Co(III)L-(CO22-)](+) complex (L = HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11- tetraazacyclotetradeca-4,11-diene; S = solvent) in the catalytic system. The electron-transfer rate constant for the reaction of p-terphenyl radical anion with Co(II)L(2+) is 1.1 x 10(10) M(-1) s(-1) and probably diffusion controlled because of the large driving force (similar to+1.1 V). Flash photolysis studies yield a rate constant 1.7 x 10(8) M(-1) s(-1) and an equilibrium constant 1.1 x 10(4) M(-1) for the binding of CO2 to Co(I)L(+) in the catalytic system. These are consistent with those previously obtained by conventional methods in acetonitrile. Studies of catalytic systems with varying cobalt macrocycles highlight some of the factors controlling the kinetics of the photoreduction of CO2. Steric hindrance and reduction potentials are important factors in the catalytic activity for photochemical CO2 reduction.