A 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS)-H2O2-peroxidase system was involved to study the oxygen reduction reaction (ORR) selectivity in acid medium. The average number of electron transferred per oxygen molecule (n) was calculated by measuring the quantity of H2O2 produced during the oxygen reduction reaction, taking advantage of the high activity of peroxidase enzyme coupled with the ABTS chromogen. The method was validated using electrode materials for which the electron pathway is established. It is simple, accurate and can be used on various kind of electrode structure.