A new polypyrrole-carrying aza crown ether in the 3 position has been synthesized, which shows reversible complexing properties towards Co2+ ions. The stoichiometry of the cobalt aza crown ether complex varies from 1:1 to 1:2, depending on the size of the crown cavity. These modified electrodes show an interesting electroactivity towards oxygen reduction in sulfuric acid solution, which increases with film thickness. Oxygen reduction proceeds by a two-electron pathway, and the kinetics of this catalyzed reaction are analyzed by the use of cyclic and rotating disk voltammetries.