We investigated the mechanism involved in the oxygen production in the Fenton chemistry by means of density functional theory calculations. This study extends previous work in which we proposed that the Fe(IV)O2+ complex is the key active intermediate in the Fenton reaction. Here we provide a consistent picture of the entire reaction cycle by analyzing how the active species, Fe(IV)O2+, can react with hydrogen peroxide to produce O2 and regenerate the Fe2+ catalyst. These results are also relevant in view of the analogies with important enzyme-catalyzed oxidation reactions.