Abstract 1,2-Benzophenoxazine-7-one (BPO) was absorbed on graphite to give a chemically modified electrode. The electrochemical redox reactions of BPO itself are fairly reversible at coverages less than 2 × 10 −9 mol cm −2 with an E °′ of −210 mV (vs. SCE) at pH 7.0. The E °′ decreased 60 mV per pH unit from pH 1 to 11. The adsorbed BPO mediated electron transfer in the electrocatalytic oxidation of the dihydronicotinamide adenine dinucleotide (NADH). The kinetic reaction was studied with a rotating disk electrode modified with BPO for different coenzyme concentrations and pHs. The formation of a charge-transfer complex between NADH and the mediator was indicated. The rate of reaction increased as the pH decreased; the slope of a log k +2 vs. pH plot was −0.55, indicating a complex proton dependence. A reaction scheme with two parallel paths is discussed.