Preparation of derivatives of benzene containing CF.sub.3 groups from the corresponding compounds containing CCl.sub.3 groups (e.g. trichloromethyl benzene) is carried out continuously with excellent yield rates by reaction with liquid hydrogen fluoride in an autoclave above 80.degree.C, (which latter exceeds the stoichiometric proportion by at least 25 mole %). The reactants are fed continuously into the autoclave. The reaction mixture preferably becomes pressurized to at least 20 atmospheres and homogeneous after having reached a certain filling level. The reaction mixture is thereafter withdrawn proportionally to its feeding-in rate. The mixture may be conveyed through one or several additional autoclaves with the final withdrawal of the reaction mixture being out from the terminal autoclave of this cascade. The fluorination products are well known starting and inermediate products for the preparation of dyestuffs and pesticidal compositions.