The fluorination of chromium oxides reduces the chromium species on the surface. A low fluorination by HF does not modify the total amount of reversibly oxidizable chromium sites, but increases the strength of these sites. Mobile dioxygen (measured by isotopic exchange) was required to obtain a good fluorination activity. These dioxygen species would be exchanged by HF which is probably active in the fluorination of CF3CH2Cl and would prevent the irreversibly catalyst fluorination.