A process for the manufacture of aromatic compounds containing fluorine comprises reacting an aromatic compound containing one or more chlorine atoms bound to the nucleus and, in addition to chlorine, at least two strongly electro-negative substituents, at least one of which is, in the case of compounds of the benzene or naphthalene series, in ortho- or para - position (for naphthalene compounds "ortho-" includes 1 : 8-position and "para-" includes 2 : 5 position) to a chlorine atom, with dry potassium fluoride at a raised temperature and in absence of a solvent. The electro-negative substituents may be nitro, sulphonamido, sulpho-ester, carboxy-ester, carbamido, carboxyanhydride or trifluoromethyl groups or quinone oxygen (in the case of anthraquinones the chlorine atom or atoms may be in either the a - or b -positions). The process may be carried out by heating in a closed vessel to a temperature between 175 DEG and 275 DEG C., using appropriate quantities of potassium fluoride to partially or wholly replace chlorine atoms by fluorine atoms. New compounds prepared by the process are dinitro-chloro-difluorobenzenes from 1 : 3-dinitro-2 : 5 : 6-trichlorobenzene and from 1 : 3 - dinitro - 2 : 4 : 6 - trichlorobenzene; 1 : 3 - dinitro - 4 - fluoro - 5 - trifluoromethylbenzene from 1 : 3 - dinitro - 4 - chloro - 5 - trifluoromethyl - benzene; 2 - chloro - 3 - fluoronaphthoquinone from 2 : 3-dichloro-naphthoquinone; 1 - fluoroanthraquinone from 1-chloroanthraquinone; 1 : 8 - difluoroanthraquinone from 1 : 8 - dichloroanthraquinone; 2 : 5 - difluoro - 3 : 6 - dichloro - 1 : 4 - benzoquinone and 2 - fluoro - 3 : 5 : 6 - trichloro-1 : 4 - benzoquinon from chloranil; and 1-fluoro - 2 - nitrobenzene - 4 - sulphonic acid diethylamide from 1 - chloro - 2 - nitrobenzene-4 - sulphonic acid diethylamide. Other compounds which may be prepared by the process include 1 : 3 - dinitro - 4 : 6 - difluorobenzene and 1 : 3 - dinitro - 4 - chloro - 6 - fluorobenzene from 1 : 3 - dinitro - 4 : 6 - dichlorobenzene; 3-nitro - 4 - fluoro - benzoic acid - ethyl ester from 3 - nitro - 4 - chloro - benzoic acid ethyl ester; 3 - nitro - 6 - fluoro - benzoic acid methyl ester from 3-nitro-6-chloro-benzoic acid methyl ester; 4-fluoro-phthalic anhydride from 4-chloro-phthalic anhydride; and 2-fluoroanthraquinone from 2-chloroanthraquinone.