Olefines are polymerised by a supported Ziegler-type catalyst, made by mixing the two components of the catalyst in the presence of an inert carrier, upon which the final catalyst is found. Suitable catalysts are described in Specifications 833,430, 801,301, 799,823, 799,392 and consist of two components (A) a compound of a metal of Groups 4A, 5A, 6A, 7A or 8 of the Periodic system and (B) an organo-metallic compound or a hydride of a metal of Groups 1, 2, or 3, or of tin or lead; or an organic compound of boron or an addition product thereof with an alkali metal hydride. As inert carriers there may be used powdered silicon carbide, calcium phosphate, magnesium carbonate, or water-soluble salts e.g. alkali metal chlorides, carbonates, or sulphates, but polymeric substances are excluded. In examples, aluminium di-ethyl mono-chloride or aluminium tri-ethyl and titanium tetra chloride or tetraethoxide are added to a suspension of dry powdered silicon carbide, basic calcium phosphate or magnesium carbonate suspended in iso-octane in an inert atmosphere, in a halogenated hydrocarbon, or in light petroleum spirit containing an anti-oxidant. Ethylene is then passed in to form a polymer which is easily separated from the supported catalyst.ALSO:A catalyst for polymerising olefines of the Ziegler type, is made by mixing the two components of the catalyst in the presence of an inert carrier, upon which the final catalyst is formed. The two components of the catalyst are (A) a compound of a metal of Groups 4A, 5A, 6A, 7A or 8 of the Periodic system and (B) an organo-metallic compound or a hydride of a metal of Groups 1, 2 or 3; or of tin or lead, or an organic boron compound or an additive product thereof with an alkali metal hydride. Suitable catalyst components are described in Specifications 833,430, 801,301, 799,823, 799,392. As inert carriers there may be used, powdered silicon carbide, calcium phosphate, magnesium carbonate, or watersoluble inorganic salts e.g. alkali metal chlorides, carbonates, or sulphonates, but polymeric substances are excluded. In examples, aluminium diethyl monochloride or aluminium triethyl and titanium tetrachloride or tetra-ethoxide are added to dry basic calcium phosphate, silicon carbide, or magnesium carbonate in iso-octane, in an inert atmosphere, or in light petroleum spirit containing an anti-oxidant. Halogenated hydrocarbons may also be used. Ethylene is then passed in to form a polymer which is easily separated from the supported catalyst.