Abstract The effect of the carrier characteristics, such as specific surface area and acidity, on the activity of supported Ziegler catalyst in the ethylene polymerization has been investigated. Silica-gel and silica-alumina, containing various quantities of alumina (5, 13 and 25 wt-%) and also modified by deposition of Zn metal, wer used as carriers. The different carriers give rise to catalysts with different activities after deposition of TiCl 4 and activated with Al(C 2H 5) 2Cl. The activity in ethylene polymerization is controlled firstly by the chemical composition of the carriers and secondly by its specific surface area. The silica-aluminas show higher acidities than silica-gel and deposition of Zn is accompanied by considerable changes in the carriers, especially increasing their acidity and decreasing the specific surface areas. The increase of the carrier acidity gives rise to an increase in the polymerization activitity and in the polymer molecular weights. Coordinately unsaturated titanium ions, on the surface of the catalysts containing Zn metal, are detected by ESR, showing a paramagnetic signal similar to those produced by TiCl 3 crystals.