The reductive nitrosylation (Fe-III(P) + 2NO + H2O = Fe-II(P)(NO) + NO2- + 2H(+)) of the ferriheme model Fe-III(TPPS) (TPPS = tetra(4-sulfonatophenyl)porphyrinato) has been investigated in moderately acidic solution. In the absence of added or adventitious nitrite, this reaction displays general base catalysis with several buffers in aqueous solutions. It was also found that the nitrite ion, NO2-, is a catalyst for this reaction. Similar nitrite catalysis was demonstrated for another ferriheme model system Fe-III(TMPy) (TMPy = meso-tetrakis(N-methyl-4-pyridyl)porphyrinato), and for ferriheme proteins met-hemoglobin (metHb) and met-myoglobin (metMb) in aqueous buffer solutions. Thus, it appears that such catalysis is a general mechanistic route to the reductive nitrosylation products. Two nitrite catalysis mechanisms are proposed. In the first, NO2- is Visualized as operating via nucleophilic addition to the Fe-III-coordinated NO in a manner similar to the reactions proposed for Fe-III reduction promoted by other nucleophiles. This would give a labile N2O3 ligand that hydrolyzes to nitrous acid, regenerating the original nitrite. The other proposal is that Fe-III reduction is effected by direct outer-sphere electron transfer from NO2- to Fe-III(P)(NO) to give nitrogen dioxide plus the ferrous nitrosyl complex Fe-II(P)(NO). The NO2 thus generated would be trapped by excess NO to give N2O3 and, subsequently, nitrite. It is found that the nitrite catalysis rates are markedly sensitive to the respective Fe-III(P)(NO) reduction potentials, which is consistent with the behavior expected for an outer-sphere electron-transfer mechanism. Nitrite is the product of NO autoxidation in aqueous solution and is a ubiquitous impurity in experiments where aqueous NO is added to an aerobic system to study biological effects. The present results demonstrate that such an impurity should not be assumed to be innocuous, especially in the context of recent reports that endogenous nitrite may play physiological roles relevant to the interactions of NO and ferriheme proteins.