The reaction of Fe-III porphyrin complexes bearing distal hydrogen bonding residues with sulfide/hydrosulfide is kinetically monitored to reveal the presence of an intermediate and a k(H)/k(D) of 3.0. This intermediate is trapped at low temperatures and investigated with resonance. Raman and electron paramagnetic resonance spectroscopy. The results, corroborated by density functional, theory calculations, indicate that this species is a six coordinate low spin hydrosulfide bound ferric porphyrin. The homolytic cleavage of the Fe-III-SH bond resulting in the formation of a ferrous porphyrin and hydrosulfide radical (trapped with 5,5-dimethyl-1-pyrrilone-N-oxide) is found to be the overall rate-determining step of the reaction.