The catalytic wet oxidation of phenol has been investigated in a 1 L semi-batch reactor in the presence of both ferrous and ferric salts. Oxidation reactions follow first-order kinetics with respect to phenol and half-order kinetics with respect to dissolved oxygen. The activation energy for the reaction was 44.5 and 48.3 kJ mol(-1) for runs employing Fe3+ and Fe2+, respectively. Rate constants and induction periods were also similar for both catalysts. This result could be explained by analysing the evolution of iron during the oxidation process. For pH > 2, Fe2+ was rapidly oxidized under reaction conditions to Fe3+, resulting in a unique catalytic redox system Fe2+/Fe3+. It was also shown that if pH < 2 the dissolved oxygen was unable to oxidize ferrous ion, resulting in a much slower oxidation rate of phenol. The absence of a redox pair resulted in a complete lack of catalytic activity of the dissolved iron salt. (c) 2005 Society of Chemical Industry.