A catalyst component is prepared by treating a magnesium halide with a Lewis Base compound and a transition metal compound and subjecting the treated productto at least one washing step using a halogenated ethylene, for example tetrachloroethlene, at a temperature of greaterthan 80°C. The transition metal compound can be titanium tetrachloride and the Lewis Base compound an ester of an aromatic carboxylic acid, for example ethyl benzoate or di-isobutyl phthalate. The treatment can be effected in the presence of a non-metallic halide such as thionyl chloride. The treatment can be effected using a milling process. The product can be used for the polymerisation of olefin monomers such as propylene.