1,2,3,4-Tetrachlorobenzene, 1,2,3,5-tetrachlorobenzene, 2,4,6-trichlorofluorobenzene, and 2,6-dichloro-1,4-difluorobenzene were fluorinated with potassium fluoride and potassium fluoride-cesium fluoride mixtures in dimethyl sulfone. By varying the concentration, temperature and reaction time, the degree of fluorination could be controlled to some extent. The optimum conditions for producing mono-, di- and tri-fluoro-substituted chlorobenzenes and trace amounts of tetrafluorobenzene from the corresponding tetrachlorobenzenes are given. 1,2,3,5-Tetrafluorobenzene was obtained in 44.8% yield from 2,6-dichloro-1,4-difluorobenzene. 1,2,3,4-Tetrafluorobenzene was obtained in only trace amounts from 1,2,3,4-tetrachlorobenzene. A total of 24 new chlorofluorobenzenes and intermediates are described. Fluorination with potassium fluoride and certain other metal fluorides was also investigated. ?? 1972.