Fe-based electrocatalysts for the reduction of oxygen in polymer electrolyte membrane (PEM) fuel cells have been prepared by adsorbing either or ions on two carbon blacks to determine if their maximum activity is limited by (i) the maximum number of micropores available to host catalytic sites in the support or by (ii) the maximum number of ions able to be adsorbed on the carbon. The two carbon supports having the same microporous surface area, one etched in air and the other in , were derived from the same carbon black (N234). Air-etched N234 is more acidic in nature as it possesses carboxylic functionalities that can adsorb , while -etched N234 possesses both pyridinic and carboxylic functionalities that can adsorb either or . Catalysts were prepared by heat-treating, in pure , the materials resulting from or adsorption on both etched carbons. It is concluded that, when catalysts are prepared in pure , the catalytic activity is only governed by the number of micropores, having a size between 0.8 and , that are available to host the catalytic sites in the porous volume of the carbon support, because Fe ion uptake by adsorption is never the limiting factor.