Possible use of ozone (O3) for producing higher oxide was thermodynamically analyzed. Since the activity of oxygen in ozone is about 1018 at room temperature and 108 at 773K, ozone may react with metals to yield higher oxidation state products than an ambient pressure of pure oxygen. In agreement with this thermodynamic prediction, AgO and Ag2O were synthesized experimentally by blowing a 6 vol%O3-oxygen gas mixture through a water-cooled lance. Neither Ag3O4 nor Ag2O3 was found. Slow oxygen diffusion into a metal and/or reaction products and decomposition of unstable oxides that are successively formed are obstacles to practical applications of syntheses by ozone oxidation.