A hydrogen peroxide (H2O2) micro sensor has been prepared on a 7 mu m carbon fiber microdisk, using horseradish peroxidase (HRP) immobilized in a poly(4-vinylpyridine) based redox hydrogel having [Os(bpy)(2)Cl](2+/3+) redox centers. Because of radial diffusion of the electrons through the hydrogel, the sensor’s steady state current density for H2O2 electroreduction was 0.17 mA cm(-2) at 1.0 x 10(-4) M H2O2, more than 10 times that of a 3 mm macroelectrode. With the microsensor positioned 50 mu m from the surface, H2O2 production by photogenerated hole oxidation of water and photogenerated electron reduction of molecular oxygen was monitored independently on a partially TiO2 coated ITO (indium tin oxide) glass. H2O2 was produced primarily by photoelectroreduction of dissolved oxygen on the ITO glass, with a lesser amount of H2O2 being produced by photoelectrooxidation of water on the TiO2 film.