A polarographic method to assess the scavenging capacity of a molecule for O2- is proposed. This method is based on the fact that O2- is not detected by the Clark electrode and that a scavenger competes with spontaneous dismutation of O2-. So, the reduction of O2 into O2- and the decomposition of H2O2 by catalase, releasing O2, show a biphasic kinetic. Various kinetic parameters can be used to calculate the nmol of O2- scavenged and also supply data on the reaction mechanisms (oxidation or reduction of O2-) involved in scavenging. This method presents several other advantages: scavenging capacity can be assayed without added indicators which themselves behave as scavengers (as demonstrated for NBT), the presence of scavengers which interfere with the O2- generating system (xanthine-xanthine oxidase) does not invalidate the measurements made.