This paper presents the results of slurry polymerisation of ethylene that has been initiated by an Al/Ti/Mg-alcohol supported catalyst. Special attention has been paid to the effects of hydrogen, catalyst composition and addition of an oi-olefin on catalyst efficiency, molecular weight, crystallinity, and physico-mechanical properties of the final polyolefin. The efficiency of this A/Ti/Mg catalyst, molecular weight and melting temperature of the formed High Density Polyethylene (HDPE) has been found to decrease as the partial pressure of hydrogen is increased. In parallel, crystallinity and elongation at break of HDPE increase. The composition of this Al/Ti/Mg catalyst has a marked effect on catalyst efficiency, e.g. decrease in the Ti relative content results in an improved efficiency from 86 to 7942 kg PE g(-1) Ti h(-1) as the Mg/Ti molar ratio is increased from 1 to 333. Copolymerisation with 1-octene has also been investigated. When ethylene polymerisation is carried out with this Al/Ti/Mg catalyst at a Mg/Ti ratio of 333, addition of the alpha-olefin is responsible for a decrease in the polymerisation rate, which is typical of a negative ＇comonomer＇ effect. This effect is still more pronounced in the presence of H-2. Comparison with other catalytic systems has been made in order to understand the role Flayed by the alpha-olefin and/or hydrogen on the catalyst efficiency and the physico-mechanical properties of the resulting polyolefins.