Fe-N-C catalysts are the most promising group of non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). This study focusses on two different porphyrin-based Fe-N-C catalysts and a Fe-N-C catalyst prepared from alternative precursors under S-addition. Catalysts are subjected to a wet-chemical poisoning treatment by sulfite ions $(\mathrm{SO}_{3}^{2-})$ . A mechanism for the deactivation process of the active sites is proposed. ORR activity is evaluated for the original catalysts (OC) and for the poisoned catalysts in 0.1 M H2SO4. In addition, the structural composition of the catalysts is identified by Mosbauer spectroscopy. Our results show that the sulfite ions bound irreversible to the catalysts and the catalysts lose significant fractions of their ORR activity while in Mosbauer spectroscopy a new doublet appears. Based on the results, possible models for the binding of the ambident sulfite ion to the FeN4 centers are discussed.