Quantitative investigation of the reaction of the ferri-heme model compound Fe-III(TPPS)(H2O)(2) (1) to give Fe-II(TPPS)(NO) (2) (TPPS = tetra(4-sulfonato-phenyl)porphinato) in buffered aqueous solution demonstrates a slow pH-independent reductive nitrosylation pathway in the pH range 4-6. The rate of this reaction is subject to modest general base catalysis. In the course of this study, a surprising catalytic pathway whereby nitrite ion (NO2-) strongly catalyzes the reduction of 1 to 2 under reductive nitrosylation conditions was demonstrated.