Perchlorofluorobenzenes, such as liquid trichlorotrifluorobenzene isomer mixtures, are prepared by heating a mixture of two or more halobenzenes selected from the group consisting of hexachlorobenzene, hexafluorobenzene and perchlorofluorobenzenes in which the number of chlorine or fluorine atoms in the components of the mixture differ by at least two, such as a mixture of dichlorotetrafluorobenzene and tetrachlorodifluorobenzene, at a temperature of at least about 475.degree.C. for a period of ...