Abstract: The invention relates to a process for the preparation of 1,1,1,2-tetrafluoroethane from 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride in the gas phase, in which a catalyst containing chromium and magnesium is used which is obtainable by precipitating chromium(III) hydroxide by reacting 1 mol of a water-soluble chromium(III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, the reaction mixture is made into a paste containing chromium hydroxide and a magnesium salt, and the paste is then dried and treated with hydrogen fluoride at temperatures of 20 to 500° C.