Ozone gas in an ambient pressure was blown to Cr2O3 powder. The higher oxides, CrO3, Cr2O5, Cr5O12, and Cr3O8, could be synthesized above 373 K. Both CrO3 and Cr2O5 were found by in situ X-ray diffraction measurements. It was known that they could be produced only at high pressure of oxygen and at high temperatures. The CrO3 formed was melted at 470 K in an ozone atmosphere and decomposed above 500 K. Because of thermal decomposition of ozone and the higher oxides, no higher oxides were formed above 660 K. The phase formation was discussed using the kinetic stability diagram.