Abstract: An olefine polymerisation catalyst comprises a titanium trichloride component of high surface area, an organo-metallic component, a Lewis Base component and optionally a polyene. The TiCl.sub.3 component which may have a high porosity is conveniently obtained by the reaction of TiCl.sub.4 and an organo-aluminium compound, treating the reaction product with a complexing agent and then contacting this product with TiCl.sub.4. The organo-metallic component is preferably aluminium triethyl, the Lewis Base component hexamethylphosphoric triamide and the polyene is cycloheptatriene. This system has high polymerisation activity combined with good stereospecificity. Polypropylene can be obtained in high yield thereby giving low catalyst residues and the polymer has a high stiffness modulus.