Hydrogen peroxide sensing microelectrodes for scanning electrochemical microscopy (SECM) have been developed and used to measure hydrogen peroxide in the diffusion layer during electrochemical reduction of oxygen on gold and carbon electrodes. These microelectrode biosensors were also used to detect immobilized glucose oxidase through the production of hydrogen peroxide during enzymatic oxidation of glucose. Images of hydrogen peroxide concentration profiles near a platinum microdisk during catalytic decomposi-tion of peroxide and in the diffusion layer of a carbon-platinum composite electrode during ox-ygen reduction are presented. The factors limiting the spatial resolution (tens of micrometers) and potential applications of the technique are discussed.