We report on the electrocatalytic activity of immobilized coenzyme B-12 and vitamin B-12 (as aquocobalamin) for the electrooxidation Of L-cysteine and their effects on the electrochemical reversibility of the L-cysteine/L-cystine redox couple, a crucial biological system. Cyclic voltammograms of coenzyme B-12 adsorbed on a graphite electrode show that upon the reductive elimination of the 5'-deoxyadenosyl group from the cobalt center, at approximately -1.1 V, the electrochemical response of the modified electrode becomes similar to that of aquocobalamin. The electrochemically pretreated coenzyme B-12 shows a high electrocatalytic activity for the electro-oxidation Of L-cysteine at physiological pH that has never been observed before with the commonly used metallophthalocyanine catalysts. Also, its activity is slightly higher than that exhibited by aquocobalamin.