Water-soluble cobalt tetrakis(N-methyl-2-pyridyl)porphine (Co(2-TMPyP)) has been investigated for their catalytic behavior toward nitric oxide reduction. The acid dissociation constants (pK(a)) for [Co-III(2-TMPyP)(H2O)(2)](5+) are 5.5 and 9.6. Nitrite anion coordinates to the Co(III) center in pH 4-12 buffer solutions. The nitrite complex [Co-III(2-TMPyP)(NO2-)](4+) exhibits IR bands at 1401, 1306 and 83O cm(-1), indicating a cobalt-nitro complex. The 5-coordinated cobaltous porphyrin, [Co-III(2-TMPyP)(H2O)](4+), has been identified by ESR and has pK(a) = 13.0. This is the first case that the pK(a) of a Co(II) porphyrin is presented. Nitric oxide from the disproportionation of nitrite anion in acidic solution can coordinate to the Co(II) center. Resonance Raman spectrum of [Co-II(2-TMPyP)(NO)](4+) exhibits a peak at 1722 cm(-1), which disappears upon exposure to air. Electrocatalytic reduction of [Co-II(2-TMPyP)(NO)](4+) at -0.3 V in pH 4.0 buffer solution generates nitrous oxide-in a slow rate. Bulk electrolysis was also conducted at -0.7 V, at which [Co-I(2-TMPyP)](3+) reacts with nitrite anion and nitric oxide. The reduction products are mainly ammonia and hydroxylamine.