Organic compounds containing a mercapto group or a dithiane moiety can be transformed into the analogous fluoro or difluoro compounds by reacting them with a halogenating reagent in liquid hydrogen fluoride, optionally containing boron trifluoride or antimony pentafluoride solution, at temperatures from about -80.degree. and 15.degree. C. The method may be descriptively termed "fluorodesulfurization".