The optimization of the synthesis of Fe/N/G catalysts for the oxygen reduction reaction (ORR) has been investigated. The synthesis of Fe/N/G comprises a solid state reaction of the precursors in a planetary ballmilling followed by a thermal treatment in ammonia/nitrogen atmosphere. First, the influence of the concentration of NH3 during the pyrolysis step was studied. The results obtained show that the influence of the NH3 concentration during the pyrolysis step on ORR activity is more critical in acid medium. Next, the effect of the nature of the iron precursor in the structure and catalytic performance of the catalysts obtained was evaluated. Representative iron precursors containing Fe atoms in different oxidation states and containing or not heteroatoms (N or S) into their structure have been studied. The catalyst displaying the highest activity for the ORR in alkaline and acid electrolyte is the one obtained when iron phthalocyanine was used as iron precursor. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.