Ultrafine particles of RuO2-IrO2 binary oxides with various compositions of Ru and Ir have been prepared by a sol-gel process consisting of hydrolysis and condensation of metal ethoxide ethanol solutions. The (Ru, Ir)O2 rutile-type solid solution of binary oxides was formed. High-resolution scanning electron micrographs of the Ru-Ir binary oxides showed the spherical-like crystallites for Ru-rich oxides and needle-like ones for Ir-rich oxides. The particle size was the smallest for the binary oxide with comparable Ru-Ir composition.