There are several known processes for manufacturing the commercially important anaesthetic desflurane (CF 3 CHFOCHF 2 ) by the catalyzed reaction of commercially available isoflurane (CF 3 CHClOCHF 2 ) with hydrogen fluoride. The present available methods have the disadvantage of high catalyst usage, with consequent environmental problems, or of having to trade off low conversion against low selectivity. An alternative catalyst system was therefore sought that would avoid these problems, but would still give the benefits of a vapor-phase process and, in particular, a long catalyst life. A catalyst consisting of antimony pentafluoride supported on activated carbon has now been found to provide the basis for a novel vapor-phase process for the fluorination of isoflurane to desflurane using hydrogen fluoride. The process operates with a long catalyst life, high conversion, and high selectivity.