The role of nitrogen and iron in the generation of catalysts for oxygen reduction in acidic media has been investigated by using two independent organic precursors. The Fe and N precursors were polyvinylferrocene adsorbed on carbon black and acetonitrile vapor, respectively. These precursors were pyrolyzed at 1000 degrees C. A catalyst is obtained only if Fe and N are present together in the reactor during pyrolysis. Inactive Fe clusters surrounded by a protective graphitic envelope are produced when adsorbed polyvinylferrocene is pyrolyzed alone at 1000 degrees C. The latter material may, however, be activated by a second pyrolysis in acetonitrile vapor. The characterization of the catalyst indicates that the iron is oxidized (Fe-II and Fe-III), but no strong Fe-N-x bonds were unequivocally detected by ToF SIMS. Lifetime testing of the catalyst in a polymer electrolyte fuel cell demonstrated stable currents for at least 300 h. The current density measured with a catalyst containing 1 wt% Fe was about 1/3 of that measured with a commercial Pt-based catalyst containing 2 wt% metal loading.