CNx catalysts were synthesized using Fe-doped MgO as a growth substrate. The oxygen reduction reaction (ORR) performance of the synthesized samples was evaluated in a half-cell. Acid-washing improved the ORR activity for the synthesized CNx samples. This is attributed to the removal of the exposed metal, which is likely to be blocking the active sites. Characterization experiments suggest that acid-washing totally eliminates the support material (MgO) and renders carbon more graphitic. This is likely to be due to elimination of amorphous or less crystalline species during washing. In addition, characterization experiments reveal significant differences in the metallic species that are exposed versus those encased in the carbon nanostructure.