The feedback mode of the scanning electrochemical microscope (SECM) was used for studying a surface-catalyzed electron-transfer reaction. As a model system we examined the reduction of protons by reduced N,N′-dimethyl-4,4′-bipyridinum, methyl viologen radical cation, MV+•, catalyzed by a platinum surface. The experimental configuration comprises a biased amalgamated Au ultramicroelectrode (UME) that approaches an unbiased Pt UME substrate of the same radius. MV+• that is electrochemically generated at the amalgamated UME is oxidized at the Pt surface as a result of reducing protons. This novel configuration permits the simultaneous measurement of the catalytic current and the mixed potential (open-circuit potential) attained at the Pt surface. The results are interpreted by an electrochemical model which was previously suggested; however, our approach makes it possible to measure, for the first time, the current-potential relation predicted by the theory. Our results indicate that methyl viologen behaves reversibly under all the experimental conditions, and that the rate constant of hydrogen evolution, that is calculated, is in perfect agreement with previous reports. The significant advantages of our approach are discussed as well as its applicability for studying other electrochemical systems.