Polymerization catalysts comprise a solution in an inert solvent of the reaction product of a tetravalent titanium compound with an alkyl aluminium compound or an alkyl tin compound, the solution containing dissolved titanium of valency below 4 in an amount less than the equivalent of 0.5 to 1.5 millimols. trivalent titanium per litre of solvent. A metal halide may also be present in the reaction, or the alkyl aluminium or alkyl tin compound may contain halogen. The amount of lower valency titanium present is kept below the above-mentioned limit to prevent the formation of a precipitate in the solution, by using an alkyl aluminium compound containing less than 1% of aluminium hydride compounds; by saturating a solution of the alkyl aluminium compound with an olefin before adding the titanium compound; by reacting hydride groups present with oxygen, with alcohols, with water, with an anhydrous hydrohalogen acid, or an aluminium halide; or by using as the alkyl aluminium compound a mono- or di-alkyl aluminium halide, alkoxide, aroxide, or acetyl acetonate. In examples catalysts are prepared by reacting in inert solvents titanium tetrachloride with tri-isobutyl aluminium treated with (a) isobutylene, (b) isobutanol, (c) ethylene, (d) anhydrous hydrochloric acid, (e) oxygen, and (f) water; hydride-free triethyl aluminium; di-isobutyl aluminium chloride; isobutyl aluminium dichloride and diisobutyl aluminium acetylacetonate; di-isobutyl aluminium fluoride; diethyl aluminium fluoride; di-isobutyl isobutoxy aluminium and isobutyl aluminium dichloride; diethyl aluminium chloride; ethyl aluminium dichloride; ethyl aluminium bromide; tetrabutyl tin and aluminium chloride; and with di-isobutyl aluminium chloride and tri-isobutyl aluminium. Also, titanium tetrafluoride is reacted with diisobutyl aluminium chloride. Ethyl aluminium sesquibromide or aluminium chloride is included in some of the catalysts. Lists of other compounds which may be used in the catalysts are given.ALSO:Olefinic hydrocarbons are polymerized using a catalyst comprising a solution in an inert solvent of the reaction product of a tetravalent titanium compound with an alkyl aluminium compound or an alkyl tin compound, the solution containing dissolved titanium of valency below 4 in an amount less than the equivalent of 0.5 to 1.5 millimoles trivalent titanium per litre of solvent. A metal halide may also be present in the reaction, or the alkyl aluminium or alkyl tin compound may contain halogen. The amount of lower valency titanium present is kept below the above-mentioned limit to prevent the formation of a precipitate in the solution and to obtain improved polymers, by using an alkyl aluminium compound containing less than 1% of aluminium hydride compounds; by saturating a solution of the alkyl aluminium compound with an olefin before adding the titanium compound; by reacting hydride groups present with oxygen, with alcohols, with water, with an anhydrous hydrohalogen acid, or an aluminium halide; or by using as the alkyl aluminium compound a mono- or di-alkyl aluminium halide, alkoxide, aroxide, or acetyl acetonate. In examples, olefinic hydrocarbons are polymerized by passing them into catalyst solutions prepared by reacting in inert solvents titanium tetrachloride with triisobutyl aluminium treated with (a) isobutylene, (b) isobutanol, (c) ethylene, (d) anhydrous hydrochloric acid, (e) oxygen, and (f) water; hydride-free triethyl aluminium; diisobutyl aluminium chloride; isobutyl aluminium dichloride and diisobutyl aluminium acetylacetonate; diisobutyl aluminium fluoride; diethyl aluminium fluoride; diisobutyl isobutoxy aluminium and isobutyl aluminium dichloride; diethyl aluminium chloride; ethyl aluminium dichloride; ethyl aluminium bromide; tetrabutyl tin and aluminium chloride; and with diisobutyl aluminium chloride and triisobutyl aluminium. Also, titanium tetrafluoride is reacted with diisobutyl aluminium chloride. Ethyl aluminium sesquibromide or aluminium chloride is included in some of the catalysts. The compounds polymerized are ethylene, isoprene and propylene. Lists of other compounds which may be used in the catalysts are given.