Activities of a series of oxide supports and dozens of supported metal catalysts toward the decomposition of aqueous ozone have been investigated. Under reaction conditions, active carbon (AC) showed comparatively high activity, while zeolite (HY and mordenite), Al2O3, SiO2, SiO2-Al2O3 and TiO2 showed no or negligible activity. Noble metals showed high activity. Among four kinds (Al2O3, SiO2, SiO2-Al2O3 and TiO2) of supported noble metal catalysts, SiO2 supported noble metal catalysts showed the highest activity. A 3% Pd/SiO2 was found to be the most effective catalyst among the tested catalysts. The order of the reaction on 3% Pd/SiO2 was 1 with respect to O-3 concentration. Apparent activation energy of the reaction on 3% Pd/SiO2 was about 3 kcal mol(-1). The reaction on Pd/SiO2 was suggested to be diffusion controlled. The effect of a catalyst on this reaction was discussed.