The pH dependence of the evolution of O-2 in the Fenton reaction has been studied by using an indirect method for measuring [O-2]. The results confirmed observations of previous workers, namely, that the evolution of O-2 diminished as the pH was lowered. A quantitative analysis of the dependence of [O-2] and [Fe2+] during the reaction as a function of time showed that the pH dependence of the course of the reaction can be accounted for by assuming an acid base reaction of the active intermediate: FeO2+ + H+ reversible arrow FeOH3+. FeOH3+ is the active entity in the reaction with Fe2+ to produce Fe3+ ions, while the other form FeO2+ is involved in reactions leading to O-2 evolution.