Iron and cobalt tetraphenylporphyrins (FeTPP and CoTPP, respectively) have been adsorbed on carbon black (C). The resulting FeTPP/C and CoTPP/C were heat-treated in Ar at various temperatures ranging from 100-1100 degrees C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. The catalysts have been characterized by XRD, XPS, ToF-SIMS, and bulk analyses. Their electrocatalytic properties have been evaluated by rotating disk electrode (rde) and gas diffusion electrode (gde) measurements. The highest rde activities at 0.70 V vs nhe were recorded for CoTPP/C and FeTPP/C heat-treated in the 500-700 degrees C range. In that temperature range, the increased catalytic activity originates from the well dispersed N-4-metal moiety or from fragments of the original molecule still containing the metal bound to nitrogen. Short term stability tests on the initially most active catalysts revealed the instability of these catalysts in comparison to those obtained at higher pyrolysis temperatures (900-1000 degrees C). At these pyrolysis temperatures the active site of the catalysts is inorganic in nature. The presence of iron and cobalt in their metallic states in these catalysts has been confirmed using XRD. TEM of the catalysts pyrolyzed at these higher temperatures revealed that most of the iron and cobalt are encapsulated in a graphite-like coating. The better stability of these catalysts in the acidic environment of a working fuel cell may be ascribed to the presence of this protective coating. The activity of FeTPP/C pyrolyzed at 1000 degrees C was observed to increase with time in a manner similar to that of platinum-based catalysts. The activity of the former catalyst was determined to be approximately one half that of a commercial platinum catalyst containing the same amount of metal (2 wt%).