The H+/H-2, redox couple was investigated as a mediator system for scanning electrochemical microscopy (SECM) with proton reduction from a 0.01 M HClO4 solution at a Pt tip. The feedback behavior of the mediator was examined at different substrates (Pt, Au). Unlike the one-electron outer-sphere redox couples usually used as mediators in SECM, this mediator system is sensitive to the catalytic activity of the substrate surface for hydrogen oxidation rather than the conductivity of the substrate. The catalytic oxidation of H-2 on Pt was studied during cyclic voltammetric scans of the Pt substrate with a tip close to the substrate, and the inhibition by oxygen adsorption on Pt was investigated. The quantitative H-2 oxidation rate was determined at different substrate potentials from SECM approach curves. The kinetic parameters for proton reduction and hydrogen oxidation were determined by steady-state SECM voltammograms. The inhibitory effect of anions (Br-, I-) and reduction products of NO3- for catalytic H-2 oxidation on the Pt surface was also examined.