Optical and EPR spectroscopy of ferric heme proteins of the porphyrin, oxyporphyrin, and isobacteriochlorin classes has indicated that nitrite reacts with these proteins at the heme iron. Sulfite has been conclusively proven to react only with proteins containing the isobacteriochlorin macrocycle. Quantitative EPR spectroscopy of these nitrite and sulfite adducts showed that most contained a substantial quantity of undetectable heme. It is suggested that protein-induced autoreduction of nitrite (but not sulfite) and a strained and/or uniaxial g-tensor are the principal ways by which the silent state is produced.