The activity of gamma-alumina and gamma-alumina promoted with transition metal chlorides for the hydrolytic decomposition of dichlorodifluoromethane was investigated in the temperature range 250-400 degrees C, It was found that the hydrolysis of dichlorodifluoromethane occurs on the acid sites of the gamma-alumina. Carbon dioxide was the main product formed. gamma-alumina was deactivated by reaction with hydrogen fluoride formed by the hydrolysis reaction. The fresh catalysts and catalysts after the reaction were characterized by X-ray photoelectron spectroscopy and X-ray diffraction. Surface chloride was found to be replaced by fluoride during catalytic reaction. After prolonged reaction, rhombohedral aluminum fluoride was detected in the aged catalysts. The deactivated catalyst also catalyzed the formation of chlorotrifluoromethane. Transition metal chlorides were dispersed on the surface of gamma-alumina and reduced the hydrolytic activity of the fresh catalyst. Transition metal fluoride phase formed by reaction of the chlorides with HF during reaction, especially that formed from CrCl3, reduced the rate of fluorination of gamma-alumina and thus its deactivation.