We investigated the diffusion process of H2O2 generated during O-2 reduction at a Pt microdisk electrode used as a generator for scanning electrochemical microscopy (SECM). First, the amount of O-2 consumption and generated H2O2 at the Pt generator electrode are estimated using a detector electrode installed in the SECM. Based on the results, a large amount of O-2 consumption and generated H2O2 are detected at the center of each generator electrode. According to the measurement, the O-2 starvation and H2O2 detection currents can be defined. Subsequently, the O-2 starvation and H2O2 detection currents are measured by varying the generator size. As a result, these currents decrease with a decrease in the generator electrode size, however, the H2O2 diffusion process is changed for the generator diameter of less than 50 mu m. Finally, the O-2 starvation and H2O2 detection measurements were conducted using a Nation-coated Pt microdisk electrode. The amount of O-2 consumption is not suppressed, while the amount of generated H2O2 decreases with the Nation layer prepared on the Pt electrode. This result indicates that the thickness of the H2O2 diffusion layer in the H2SO4 solution is dramatically diminished by coating the Nation layer on the Pt generator. (C) 2009 Elsevier B.V. All rights reserved.