Aliphatic halohydrocarbons having 1-4 carbon atoms of which at least one (other than a doubly-bonded carbon atom) is attached to at least two halogen atoms other than iodine, at least one of which is chlorine or bromine, and hydrogen fluoride are passed at 125-600 DEG C., preferably 150-500 DEG C., through a bed of catalyst comprising a basic chromium fluoride of formula CrO3F2 (see Group III) whereby fluorination takes place. With halomethanes and more reactive halohydrocarbons temperatures below 350-400 DEG C. are preferred. The HF/halohydrocarbon ratio may be as low as 0.5 but normally the stoichiometric amount of HF, or slightly more than that necessary to produce the desired compound is preferred. Suitable contact times are from 1-20 secs.; pressures may be sub- or super-atmospheric and for more highly fluorinated compounds pressures of 10-200 p.s.i.g. may be employed. If desired the fluorination may take place in two or more stages at successively higher temperatures, e.g. a first stage at 200 DEG C. and a second at 300-350 DEG C. Difficulty vaporizable reactants such as hexachloroethane or carbon tetrabromide may be mixed with a solvent diluent such as perchloroethylene before being passed through a heated bed of the catalyst. The reactant to be fluorinated may also contain fluorine, and when hydrogen atoms are present and there are two or more carbon atoms, the halogen atoms are preferably attached to a single carbon atom. Specific organic reactants are CCl4, CHCl3, 1,1,1-trichlorethane and the bromine analogues thereof and methylene chloride, methylene bromide, methylene chloro-bromide, dichlorofluoromethane, dibromofluoromethane, bromochlorofluoromethane, dichlorodifluoromethane, dibromodifluoromethane, bromochlorodifluoromethane, trichlorofluoromethane, tribromofluoromethane, dibromochlorofluoromethane, bromodichlorofluoromethane, unsymmetrical tetrachloroethane, hexachloroethane, 1,1-dibromopropane, 1,1,1-trichloropropane, 2,2-dichloropropane, 1 - bromo - 1 - chloro - butane, 1,1,1 - trichlorobutane and 2,2 - dichlorobutane. To prepare fluorinated haloalkanes having no more hydrogen atoms than the original reactant from unsaturated chlorohydrocarbons, chlorine is added to the hydrogen fluoride reactant, e.g. perchloroethylene may be mixed with 1/2 mol. of chlorine and at least one mol. of HF to produce the same products as would be obtained with hexachloroethane and hydrogen fluoride, viz. all products CCl3-CCl2F to CClF2-CClF2 depending on the proportion of HF. Alternatively the degree of unsaturation need not be changed if the Cl2 is omitted and compounds having at least two chlorine, bromine, or fluorine atoms, at least one of which is chlorine or bromine, attached to a carbon atom at least once removed from the doubly bonded C-atoms which are in turn attached to at least one chlorine, bromine or fluorine atom are treated; specified compounds are CHCl=CClCCl3, CF2=CClCClF2, CCl2=CFCHCl2, CF2=CFCHBr2, CHCl=CHCl3, CClF=CF-CCl2-CCl2F, CHCl=CF-CH2-CCl3, and CF2=CCl-CCl2-CH2F. Examples relate to (1) the conversion of CCl4 to a mixture of CClF3, CCl2F2, CCl3F and CCl4 under various conditions of temperature from 125 DEG to 250 DEG C. and reactant ratios from 2.3-1.3, the reaction mixture being obtained by bubbling hydrogen fluoride through liquid carbon tetrachloride at suitable temperatures; the products were scrubbed by passing through ice cold water in a polyethylene vessel, dried by an ice-cooled trap followed by anhydrous calcium sulphate, refrigerated to condense the constituents of lower boiling point and the uncondensed gases collected; (2) the conversion of CCl4 to the above products together with about 16 mol. per cent of CF4 using a temperature of 400 DEG C. and a HF/CCl4 ratio of 7.74; (3) the conversion of CCl3F to the products in (1) using a temperature of 250 DEG C. and a ratio of about 0.27; (4) the conversion of CHCl3 to CHF3 and CHClF2 at 400 DEG C. with a ratio of 3.6; (5) the conversion of CH2Cl2 to CH2F2 and CH2ClF at 250 DEG C. with a ratio of 2.8; (6) the conversion of CBr4 to CBrF3, CHF3, CBr2F2 and CBr3F at 300 DEG C. with a ratio of 8; (7) the conversion of CBr3F to CBrF3 and CBr2F2 at 250 DEG C. with a ratio of 2.58; (8) the conversion of CCl2F-CCl2F to CF3-CF3 and intermediate products at 500 DEG C. using a ratio of 2.15; (9) the conversion of CF3CCl2CCl2F mainly to CF3CCl2CClF2 at 450 DEG C. with a ratio of 2.3. Specification 468,447, [Group IV], and U.S.A. Specification 2,458,551 are referred to.ALSO:A greenish-tan, water-insoluble basic chromium fluoride of formula CrO3F2 which is amorphous to X-ray diffraction and may be used to catalyse the fluorination with hydrofluoric acid of halohydrocarbons having from 1-4 carbon atoms, is prepared by contacting a hydrated chromium fluoride such as CrF3.3H2O with a gas comprising molecular oxygen, such as air, at above 350 DEG C., preferably 350 DEG to 750 DEG C., or by heating said fluoride with a compound such as CrO3 which releases oxygen at 350-750 DEG C., preferably after pelletizing the mixture. A suitable starting material is obtained by reacting CrO3 with excess 50-70 per cent HF in the presence of e.g. formaldehyde, toluene, xylene, sugar or polyethylene to dissolve and reduce the CrO3: by drying at 90-110 DEG C., a -CrF3.3H2O is formed. This may then be ground, mixed with 2 per cent by weight of graphite and pelleted, or slurried and pasted on activated charcoal or magnesium fluoride gel as a carrier, and either product then activated or dried and activated by heating as above described to form at least 0.1 per cent, preferably 1-20 per cent by weight of the basic fluoride. When carbon is used to prepare the pellets it is desirable to burn off the carbon by carrying out the activation at e.g. 500 DEG to 650 DEG C. or even up to 900 DEG C. Prolonged heating at lower temperatures such as 200 DEG C. should be avoided since chromic oxide is thereby produced. Before use as a catalyst and after regeneration, hydrofluoric acid is preferably passed over the product: and if a halohydrocarbon has been passed over the catalyst without hydrofluoric acid or after prolonged use, the activity can be restored by burning off the carbon in a stream of air or of oxygen at 500 DEG C. In examples: (1) CrF3.3H2O (with or without 2 per cent graphite and pelleted) was charged into a 2 in. diameter nickel tube to form a bed 12-24 in. deep, and air (in some cases moist) or oxygen passed therethrough at 500 DEG or 550 DEG C.; (2) pellets of CrF3.3H2O, CrO3 (ratio 3 : 1) and graphite were simultaneously treated in a stream of nitrogen. Specification 468,447, [Group IV], and U.S.A. Specification 2,458,551 are referred to.