Pyrolyzed Fe-N-C materials are promising platinum-group-metal free catalysts for protonexchange membrane fuel cell cathodes. However, the detailed structure, oxidation and spin states of their active sites is still undetermined. 57 Fe Mössbauer spectroscopy has identified FeN x moieties as the most active sites, with their fingerprint being a doublet in lowtemperature Mössbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (E QS) for a range of FeN x structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe-N-C catalyst, while further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device and 57 Fe Mössbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeN x moieties in Fe(II) low-spin and Fe(III) high-spin states while a minor fraction of sites could exist in Fe(II) S = 1 state. From the analysis of the 57 Fe Mössbauer spectrum under external magnetic field and the comparison of calculated and measured E QS values, we assign the experimental doublet D1 with mean E QS value around 0.9 mm•s-1 to Fe(III)N 4 C 12 moieties in high spin and the experimental doublet D2 with mean E QS value around 2.3 mm•s-1 to Fe(II)N 4 C 10 moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites while D2 may correspond either to bulk sites that are inaccessible to O 2 or to surface sites that bind O 2 weaker than D1.