Dual-disk microelectrodes with interelectrode gap widths of 5 to 48 mu m were used for the measurement of the electron transfer number n(app) of dioxygen reduction. The effects of the gap width, the scan rate and the diffusion coefficient on the generation-collection properties were studied. An equation for n(app) was derived and a semi-empirical expression of the collection efficiency was obtained. On the bare platinum electrode, the reduction of dioxygen is dominated by a four-electron reaction. On the polypyrrole (ppy) film containing nanodispersed platinum particles, more H2O2 is produced and its catalytic activity varies with the conductivity of the film. The n(app) is affected by the pH value. The diffusion coefficient of H2O2 in 1.0M KOH determined by the dual-disk microelectrode is 1.29 x 10(-5) cm(2) s(-1).