Disclosed are catalyst components for the polymerization of olefins comprising the product of the reaction of a tetravalent titanium halide or alkoxy titanium halide and an electron-donor compound with a solid obtained by the reaction of a metal oxide containing surface hydroxyls, preferably together with chemically uncombined water, with an organometallic magnesium compound used in a quantity which does not cause reduction of the titanium in the subsequent reaction with the tetravalent titanium compound.