in a process for preparing 1 , 1 , 1 - trifluorochloroethane by reacting , in a gas phase , trichloroethylene and hydrogen fluoride , when the reactant gases are diluted with a gas which is inactive to the reaction , it is very easy to control a reaction temperature , and when a generated gas from the reaction of 1 , 1 , 1 - trifluorochloroethane and hydrogen fluoride is used as a diluent gas , generation of 1 , 1 - difluoroethylene is suppressed to a very low level while not influencing the reaction between trichloroethylene and hydrogen fluoride .