Superoxide reduction by thiolate-ligated [Fe-II((SN4)-N-Me2(tren))](+) (1) involves two proton-dependent steps and a single peroxide intermediate, [Fe-III((SN4)-N-Me2(tren))(OOH)](+) (2). An external proton donor is required, ruling out mechanisms involving H+ or H-atom abstraction from the ligand N-H. The initial protonation step affording 2 occurs with fairly basic proton donors (EtOH, MeOH, NH4divided by) in THF. More acidic proton donors are required to cleave the Fe-O(peroxide) bond in MeOH, and this occurs via a dissociative mechanism. Reaction rates are dependent on the pK(a) of the proton donor, and a common [Fe-III((SN4)-N-Me2(tren))(MeOH)](2+) (3) intermediate is involved. Acetic acid releases H2O2 from 2 under pseudo-first-order conditions ([HOAc] = 138 mM, [2] = 0.49 mM) with a rate constant of 8.2 x 10(-4) s(-1) at -78 degreesC in MeOH. Reduction of 3 with Cp2Co regenerates the active catalyst 1.