Cysteine sulfenic acid CysS(O)H is shown to be formed for the reaction of cysteine (CysSH) with aqueous nitrite and the water-soluble ferriheme models Fe-III(TPPS) (TPPS = mesotetra(4-sulfonatophenyl)porphyrinato) or Fe-III(TMPS) (IMPS = meso-tetra(sulfonatomesityl)porphyrinato) at pH 5.8 and 7.4. The other product is the respective ferrous nitrosyl complex Fe-II(Por)(NO) (Por = TPPS or TMPS). Analogous oxygen atom transfers (OAT) were seen when glutathione (GSH) was used as the substrate. The sulfenic acids, CysS(O)H and GS(O)H, are transient species since they react rapidly with excess thiol to give the respective disulfides, so their presence as reactive intermediates was demonstrated by trapping with dimedone and detecting the resulting adduct using LC/MS. Preliminary kinetics studies are consistent with rate-limiting OAT from a ferric nitro complex Fe-III(Por)(NO2-) to CysSH, although this reaction is complicated by a competing dead-end equilibrium to form the thiolate complex (Fe-III(TPPS)(CysS(-)).