Fe/N/C catalysts for the reduction of oxygen under the acidic conditions prevailing in PEM fuel cells have been prepared on two carbon black supports functionalized with N-bearing groups. Once loaded with 2000 ppm Fe, the functionalized carbons were pyrolyzed at 950ºC in Ar or NH3. The best catalysts were obtained under NH3. In addition to being a nitrogen precursor, NH3 also etches the carbon support increasing its microporosity. In NH3, the functionalities are rapidly etched away from the carbon support, but this once-functionalized carbon now reacts faster with NH3 than the pristine carbon. Therefore, the optimum microporosity necessary to produce the maximum catalytic activity is reached after a shorter pyrolysis time on the functionalized carbon supports than on the pristine carbons. However, the maximum catalytic activity for the oxygen reduction reaction (ORR) is the same for the once functionalized carbon as for the pristine one.