Abstract: Polymerization catalysts, particularly catalysts for the polymerization of olefins, are disclosed. The catalysts are prepared by mixing(A) an organometallic compound or hydride of a metal belonging to Groups I to III of the Mendelyeev Periodic Table; with(B) the product obtained by contacting (1) an addition product obtained by reacting a di-, tri-, or tetravalent titanium compound with an electron-donor compound, with (2) an anhydrous magnesium halide, in particular anhydrous MgCl.sub.2 or MgBr.sub.2, which is either preactivated or mixed with the Ti compound under conditions which result in its activation; or by contacting (A) with(B') the product obtained by contacting a di-, tri-, or tetravalent Ti compound with an electron-donor compound and a support consisting of an anhydrous Mg halide, in particular MgCl.sub.2 or anhydrous MgBr.sub.2, which is either preactivated or mixed with the Ti compound and electron-donor under conditions such that the Mg halide is activated.