In spite of a significant progress in their performance in recent years, the heat-treated metal-nitrogen-carbon (M-N-C) non-precious metal catalysts for oxygen reduction reaction (ORR) at the cathode of polymer electrolyte fuel cells (PEFCs) are in need of further improvement to match the activity and, especially, the stability of Pt-based nanoparticle catalysts of oxygen reduction. A better understanding of the role of individual components in M-N-C catalysts is vital for the development of more advanced formulations. In this work, using a cobalt-polypyrrole-carbon catalyst system as an example, we demonstrate that carbon originating from the organic nitrogen precursor (ONP) has different properties than the carbon support. Unlike the carbon originating from polypyrrole, the support carbon helps to enhance ORR performance but negatively impacts the stability. To the best of our knowledge, this may be the first time that the properties of the ONP-derived carbon are being differentiated from the properties of carbon in the carbon support, emphasizing the potential importance of carbon phases in ORR electrocatalysis on heat-treated M-N-C catalysts. Copyright