To improve the water electrolysis performance of unitized regenerative fuel cells, ultrafine IrO2 powder was synthesized from colloidal precursors, and an active electrode for oxygen evolution and reduction was prepared by mixing the IrO2 powder and Pt black. Analysis showed that the surface area of the synthesized IrO2 was higher than that of high-surface-area Pt black. During fuel cell operation, IrO2 increased the overpotential slightly; however during water electrolysis, the mixed Pt-IrO2 electrocatalyst had a considerably higher activity for oxygen evolution. The addition of only a small amount of IrO2 (10-30 mol %) to the oxygen electrode was sufficient for the unitized regenerative fuel cell.