Olefins are obtained by the catalytic dehydrochlorination of alkyl chlorides which contain less than twenty carbon atoms per molecule by a process wherein an alkyl chloride or a mixture of alkyl chlorides the molecules of which contain the same number of carbon atoms or do not differ from one another by more than one carbon atom, is introduced in the liquid <PICT:0694488/IV (b)/1> phase and in admixture with a hydrated aluminium silicate as catalyst into a distillation zone provided with a fractionating column and wherein the alkyl chloride-catalyst mixture is maintained at an elevated temperature preferably 180-250 DEG C., olefin and hydrogen chloride being withdrawn as overhead product from the column and a polymer or polymer mixture being withdrawn as bottoms product. The distillation zone is preferably operated in a continuous manner but the process may also be carried out batchwise. When C1-C9 alkyl chlorides are used it is necessary to use superatmospheric pressure but with alkyl chlorides containing more than 10 carbon atoms per molecule atmospheric pressure may be employed. The term "hydrated aluminium silicate" is used to signify an earth or clay of which the principal constituents are Al2O3, SiO2 and H2O, other metallic oxides often present being Fe2O3, CaO, MgO and alkali metal oxides. Specified earths or clays are Fuller's earth and bleaching earths. Polymerization by-products are drawn off from the bottom of the distillation zone whilst the olefins formed together with hydrogen chloride are withdrawn overhead from the column. When an acid activated Fuller's earth, e.g. the product known as "Tonsil" is used the polymer by-products are stated to be useful as lubricating oils. It is an advantage to use two fractionating columns connected together in series the feed to the second column being the overhead product, freed from hydrogen chloride, from the fractionating column provided on the distillation zone, the bottom product of the second column being recycled to the distillation zone or mixed with the feed to the distillation zone. If necessary, further quantities of catalyst and of hydrochloric acid combining substances such as zinc oxide or magnesium oxide are added at the bottom of the second column. When alkyl chlorides having fewer than ten carbon atoms per molecule are used they are mixed with the catalyst and passed from below into a deep layer of high molecular weight hydrocarbons heated to 200-300 DEG C., the hydrocarbons being liquid at these temperatures. These hydrocarbons preferably consist of the polymerization products formed during the dehydrochlorination and the layer of hydrocarbons may be caused to circulate in a thermosyphon circuit. Apparatus for carrying out this method is shown in the drawing, the alkyl chloride being mixed with the catalyst in vessel 1 whilst nitrogen is passed in under pressure through a pipe 2. The catalyst and alkyl chloride is heated and resulting slurry flows to the bottom of a vessel 4 which is heated to bring the hydrocarbon mixture in the pipe 5 to 200-300 DEG C., whereby the convection currents cause the hot dehydrochlorination medium to circulate round the closed circuit 4, 5, 6. The mixture of alkyl chloride and catalyst rises to the top of the hydrocarbon mixture in the pipe 5 where it vaporizes and simultaneously forms olefins and gaseous hydrogen chloride. The vapours then pass into the fractionating column 10 from which the distillate passes into a condenser 11 whilst the alkyl chlorides separated in the column together with polymer by-products flow through pipe 12 and the distillation vessel 4 to the circulating high boiling hydrocarbon mixture. Part of the condensate from the cooler 11 is refluxed to the column 10 whilst the remainder passes into a second continuously operating column 15, the alkyl chloride carried over from column 10 being returned from vessel 16 to the starting material in vessel 1. The hydrogen chloride formed is removed overhead from the cooler 11 and the final product is cooled in cooler 18 after passage through the condensing cooler 17. Excess liquid in the circuit may be removed through the valve 20. In examples: (1) a chlorinated C13 fraction (obtained by chlorinating a saturated C13 hydrocarbon fraction prepared by the hydrogenation of a C13 hydrocarbon fraction from the catalytic hydrogenation of carbon monoxide) is passed in admixture with "Tonsil" into a first distillation vessel which is at a temperature of 235 DEG C. and is provided with a fractionating column packed with glass rings and the polymer product formed is passed into a second distillation column for removal of the chlorinated C13 fraction entrained in the polymers, the second distillation vessel which is at a temperature of 245 DEG C. being connected to the fractionating column provided on the first distillation vessel; the column is operated at a reflux ratio of 1 : 10 and hydrogen chloride and a C13 olefin fraction are withdrawn overhead whilst the polymer products are withdrawn from the base of the second distillation vessel; (2) a C8 alkyl chloride fraction (obtained in a manner similar to the chlorinated fraction used in (1)) is dehydrochlorinated by the thermo-syphon apparatus described above the alkyl chloride in admixture with "Tonsil" being passed into the thermo-syphon system filled with polymerization products from a previous run in admixture with "Tonsil," (3) a C13 monochloride obtained as in (1) is mixed with "Tonsil" and the slurry is passed into a distillation vessel maintained at 230 DEG C. and provided with a fractionating column operated at a reflux ratio of 1 : 10; hydrogen chloride and a C13 olefin-containing fraction are withdrawn overhead from the column and the C13 fraction is condensed in a cooler and passed into a second continuously operating column also operated with a reflux ratio of 1 : 10, the bottoms product from this column being recycled to the first column along with fresh alkyl chloride feed whilst the high boiling polymerization products are continuously withdrawn from the distillation vessel connected to the first column. The C13 olefin is recovered as overhead product from the second column. Specification 512,463 and German Specification 565,160 are referred to.