Au wire electrodes coated with phenothiazine derivatives such as thionine, azure C, and azure A covalently bound to a self-assembled monolayer of 3,3’-dithiobis(succineimidylpropionate) on an Au substrate showed two redox waves due to monomeric and dimeric forms of the phenothiazine derivatives. It was found that the dimeric forms have high activities for electrocatalytic oxidation of NADH. The second-order reaction rate constant for NADH oxidation was evaluated by digital simulation. The rate constants obtained at the thionine-modified electrodes were about 10(4) times greater than that obtained for the oxidized phenothiazine derivatives dissolved in solution, and were greater than those reported previously with use of other electrocatalysts.