The water-soluble manganese(III) meso-tetrakis (N-ethylpyridinium-2-yl) porphyrin ((MnTEPyP)-T-III) and manganese(III) meso-(tetrakis(4-sulfonato-phenyl)) porphyrinate ((MnTPPS)-T-III) are able to chemically distinguish between HNO and NO donors, reacting with the former in a fast, efficient, and selective manner with concomitant formation of the {MnNO}(7) complex (k(on(HNO)) approximate to 10(5) M-1 s(-1)), while they are inert or react very slowly with NO donors. DFT calculations and kinetic data suggest that HNO trapping is operative at least in the case of (MnTPPS)-T-III, while catalytic decomposition of the HNO donors (sodium trioxodinitrate and toluene sulfohydroxamic acid) seems to be the main pathway for (MnTEPyP)-T-III. In the presence of oxygen, the product (MnTEPyP)-T-II(NO) oxidizes back to (MnTEPyP)-T-III, making it possible to process large ratios of nitroxyl donor with small amounts of porphyrin.