Electropolymerized cobalt salen compounds on pyrolytic graphite electrodes were first tested as oxygen reduction catalysts in aqueous solutions of various pH. N,N'-Bis(salicylidene)ethylenediaminocobalt(II) [abbreviated Co(salen)] and its analogs were polymerized on a basal plane of pyrolytic graphite electrode by electrochemical polarization from deaerated acetonitrile solutions. The cyclic voltammetry of Co(salen) modified electrodes showed that electropolymerized Co(salen) effectively catalyzed oxygen reduction in aqueous solutions, and features such as half-wave potentials or limiting current density of oxygen reduction were satisfactory as compared with those of cobalt porphyrin-modified electrodes. The characteristics of oxygen reduction catalyzed by electropolymerized Co(salen) were investigated using rotating ring-disk electrode measurements in acidic, neutral, and basic solutions. The efficiency of four-electron reduction of oxygen was found to be best in basic solutions. The results revealed that although in acidic media the catalyst performance deteriorated in a short time, the performance in neutral and basic media was relatively good as compared with that of platinum, the best catalyst for this reaction.