The irradiation (λ = 313 nm) of aqueous [Co(III)(NH3)5TSC]2+(TSC-= trans-4-stilbenecarboxylate) leads to a redox reaction with formation of Co2+ as has been shown previously. The TSC-ligand is oxidized to the radical in the primary photoreaction. At higher complex concentrations (～10-2 M) this TSC radical is oxidized to stable products (Φ = 0.09 ± 0.02) by [Co(NH3)5TSC]2+,which is reduced to Co2+ leading to release of the TSC- ligand (Φ = 0.07 ± 0.02). Co2+ is thus formed by both the secondary and the primary photo chemical reaction (Φ overall = 0.16). At low complex concentrations (< 10-4 M) in the presence of air, the primary TSC radical is scavenged by O2. Benz-aldehyde is an oxidation product. This reduces the formation of Co2+ to Φ = 0.09 ± 0.02, which is taken to be the efficiency of the primary photo chemical step. It is suggested that the photoredox reaction is initiated by an electron transfer from the first excited singlet of the TSC- ligand. As shown by sensitization experiments, the population of the lowest TSC-triplet leads to the usual trans/cis isomerization of the TSC-ligand, but not to the redox reaction.