Abstract: A process for the polymerization of ethylene or the copolymerization of ethylene with an alpha-olefin, in suspension or in gas phase, in the presence of a catalyst consisting essentially of a cocatalyst and a catalytic component based on at least Mg, Ti and Cl, with the catalytic component being constituted of at least one derivative of magnesium and one chlorinated derivative of titanium in its form III and/or IV, which was subjected, prior to its use, to a reduction treatment and then to treatment with a chlorinated transition metal compound, with the said catalytic component being characterized in that the magnesium derivative is in the form MgCl.sub.2.MgO.

Abstract: A process for the manufacture of MgCl.sub.2.MgO with narrow granulometric distribution by treatment of hydrated MgCl.sub.2 with an alcohol followed by heat treatment after partial elimination of the alcohol, characterized in that the hydrated MgCl.sub.2 is suspended in the alcohol, and that the molar ratio of the alcohol to the hydrated MgCl.sub.2 is lower than the solutility ratio of these two substances at the temperature of the suspension. The resultant MgCl.sub.2.MgO can be employed as a support in catalytic components for the polymerization of olefins.