Pt-group metal (PGM)-free catalysts of the Me-N-C type based on abundant and inexpensive elements have gained importance in the field of oxygen reduction reaction (ORR) electrocatalysis due to their promising ORR-activities. Their insufficient stability, however, has fueled the interest in obtaining an in-depth understanding of their composition, which requires highly sensitive techniques compatible with their low metal contents (typically <5 wt%). In the particular context of iron-based materials, Fe-57-Mossbauer spectroscopy is often used to provide such compositional information, but requires (partially) Fe-57-enriched precursors. As a consequence, the extrapolation of conclusions drawn from Mossbauer measurements on Fe-57-enriched catalysts to equivalent materials with the standard isotope distribution relies on the assumption that the metal precursor's isotopic profile does not affect the catalysts' composition and ORR-activity. To verify this hypothesis, in this study we prepared two series of Fe-based catalysts using distinctively different synthesis approaches and various relative contents of Fe-57-enriched precursors, and observed that the extent of the latter parameter significantly affected the catalysts' ORR-activity. This effect was successfully correlated with the Fe-speciation of the catalysts inferred from the characterization of these samples with Mossbauer and X-ray absorption spectroscopies. Ultimately, these results highlight the crucial importance of verifying the consistency of the catalysts' activity and composition upon comparing standard and Fe-57-enriched samples.