Electrochemistry and Raman spectroscopy have shown that aquocob(III)alamin (CbI(III)) can be reduced by nitric oxide (NO) to form CbI(II) on an electrode surface. The CbI(II) formed in this way can bind NO to form nitrosylcobalamin, Cbl(II)-NO, which is reduced to form Cbl(l) at about -1.0 V vs a KCl saturated Ag/AgCl reference electrode. In addition, nitrite was found to bind both CbI(III) and CbI(II) and a binding constant of 3.5 x 10(2) M-1 was measured for (NO2-Cbl(II))(1-). UV-vis spectrophotometry and mass spectroscopy were used to show that Cbl(I) reduces NO to form Cbl(II)-NO and N2O and N-2, and this reaction is involved in the cyclic voltammetry of cobalamin in the presence of excess NO where a catalytic reduction of NO occurs involving the cycling of Cbl-(II)-NO/Cbl(I). This redox couple is also involved in the electrochemical catalytic reduction of nitrite, These results can be used to explain a number of physiological effects involving NO interaction in biological systems with added cobalamin or with cobalamin in the methionine synthase enzyme.