Ultrathin chromium oxide films were prepared on a W(100) surface under ultrahigh-vacuum conditions and investigated in situ by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy electron diffraction. The results show that, at Cr coverage of less than 1 monolayer, CrO2 is formed by oxidizing pre-deposited Cr at 300-320 K in similar to 10(-7) mbar oxygen. However, an increase of temperature causes formation of Cr2O3. At Cr coverage above I monolayer, only Cr2O3 is detected.