Density functional theory (DFT) calculations have been performed to investigate the benzyl alcohol oxidation to acetaldehyde by a Cu^II/L–TEMPO (L: bis[2-N-(4-fluorophenyl)-pyrrolylcarbaldimide]) catalyst system in alkaline water solution. The catalytic cycle consists of catalyst activation, substrate oxidation and catalyst regeneration parts. Four pathways (path A, path B, path C and path D) for the catalyst activation step are considered. The rate-determining steps are proton transfer in these pathways. Moreover, water participation plays a major role in the proton transfer step. In consideration of the high concentration of the solvent water, path C and path D are supposed to be competitive during the reaction, with path D more favourable.