The catalytic activity of chromium (III) oxide for the fluorination of CF3CH2Cl (HCFC 133a) is proportional to the number of reversibly oxidized sites. The proportionality coefficient depends on the atmosphere employed during the pretreatment of the catalyst. The temperature-programmed reduction and temperature-programmed oxidation experiments constitute a simple technique that allows the number of reversibly oxidized chromium atoms to be measured. The method of preparation of the chromium hydroxide has little effect on the catalytic properties of chromium (III) oxide. The activation atmosphere and the temperature are essential parameters in the formation of chromium (III) oxide from hydroxide. Indeed, the most active chromium (III) oxide for the fluorination of CF3CH2Cl is obtained by thermal treatment of hydroxide at 380 degrees C under nitrogen.