Abstract The steady-state production of oxygen catalyzed by fungal Arthromyces ramosus peroxidase (ARP) was investigated in the pH range from 4 to 10.6. The reaction was mediated by 1-( N,N-dimethylamine)-4-(4-morpholine) benzene (AMB), 2,2′-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS) and 1,2,4,5-tetramethoxybenzene (TMB), having low (0.39 V), medium (0.7 V) and high (0.9 V) redox potentials, respectively. AMB mediated oxygen production was indicated at pH > 7, ABTS acted at pH ≥ 7 and TMB was active at pH < 7. For evaluation of a mechanism of the pseudocatalatic reaction the mediators oxidation rate was measured at various pH. When AMB and ABTS were used the ARP activity decreased at pH > 8.5 and p K a of transition was 9.6 and 9.5, respectively. The TMB oxidation rate reaches a maximum at pH 5.3. The rate change at pH 4–5 corresponding to a single proton transfer with p K a 5.0 and an activity decrease at pH 5.5–7 corresponding to p K a 5.6. Analysis of the experimental results showed that oxygen was produced in a consecutive process including enzyme mediator oxidation and following chemical reaction of the oxidized mediator with hydrogen peroxide or its dissociated form (HO 2 −). Depending on pH the oxygen production rate was limited by the chemical or the enzymatic reaction. In the range pH 4–7 oxidized TMB reacted with hydrogen peroxide (H 2O 2), and the process was limited by the enzymatic reaction. At pH 7 and in the more alkaline area the cation radical of ABTS reacted with H 2O 2 and its dissociated form. Oxidized AMB reacted with HO 2 − at pH > 7. The oxygen production rate was correlated with the reactivity of HO 2 − estimated from the Marcus cross relationship (Marcus and Sutin, Biochim. Biophys. Acta, 811 (1985) 265).