Abstract: A process for fluorinating a hydrocarbon substrate is by mixing it with an inert diluent and mixing the diluted substrate with a gaseous fluorinating reagent to form a foam reaction mixture. The fluorinating reagent reacts with the substrate in the form to foam product. The foam is separated into a liquid component which contains product and a gas component. Product is withdrawn from the liquid component which is then returned to the diluting step. Fluorinating reagent is fed into the gas component which is then recirculated to the mixing step. An installation for fluorinating a hydrocarbon substrate includes, in sequence as a circuit, a dilution stage, a mixing stage, a reaction stage and a separation stage. Liquid returning means feeds liquid from the separation stage to the dilution stage, and gas recirculation means feeds gas from the separation stage to the mixing stage.