This paper reports on the catalytic behaviour of cyanocobalamin (VB12) towards the reduction and oxidation of nitric oxide. When VB12 is adsorbed on glassy carbon electrodes, it catalyses the reduction of nitric oxide (NO) in pH 4 and 9 buffers. In the absence of NO, cyclic voltammetry shows that VB12 is reduced by a one-step two-electron reduction from Co-III to the Co-I species. Addition of NO at pH 9 to solutions of VB,, resulted in the splitting of the cyclic voltammetry peaks as a result of a consecutive one-electron reduction of the central Co-III metal in VB12 to Co-II and finally to Co-I. Th, catalytic peak for oxidation of NO on a glassy carbon electrode modified with VB12 was observed at 1.71 V versus Ag\AgCl. at pH 9. The products of the catalytic reduction of nitric oxide include ammonia and hydroxylamine. (C) 2000 Elsevier Science Ltd. All rights reserved.