The reduction of oxygen at electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy), polythiophen (PTh), and poly(3-methyl)thiophen (PMeT), poly (3,4-ethylenedioxythiophene) (PEDOT) have been studied. The electrochemical investigations of ECPs based catalytic electrodes in oxygen-saturated electrolytes indicate the existence of electrocatalytic activity toward the oxygen reduction at different ECPs with the exception of PEDOT. To explain the reasons of the catalytic activity of ECPs, a quantum-chemical modeling of ECPs and adsorption complexes of ECPs with oxygen have been performed. To calculate the electronic structure of molecular clusters and their complexes with oxygen, the PM 3 program of the MOPAC computer system was used. The catalytic activity takes place due to the unique electronic structure of ECPs. The calculations showed that the bond orders in chemisorbed oxygen molecules at PANI decrease by a third, and the bond length increases by more than 20% in comparison with that in a free oxygen molecule. Thus, chemisorbed oxygen molecules have a fairly high degree of activation and can be readily reduced at the polymeric surface. The similar mechanism takes place on the active carbon atoms of PPy, PTh, and PMeT. (C) 2004 Elsevier Ltd. All rights reserved.