A process for the manufacture of a modified titanium component for catalysts of the Ziegler/Natta type by milling a mixture of a titanium compound of the formula TiCl.sub.3 .13AlCl.sub.3 and an organic electron donor containing phosphorus and/or nitrogen atoms. The characterizing feature is that the mixture is first milled wet in a liquid inert hydrocarbon medium under specific conditions, then separated from the hydrocarbon medium and finally milled dry under specific conditions. Such modified titanium components give catalysts which provide particularly well reproducible results in the polymerization of .alpha.-olefins.