The enhancement of the activity and productivity of heterogeneous V-based catalysts in the presence of halogenated additives, has been investigated for high-temperature ethylene homopolymerization (160-degrees-C). The influence of both the nature and structure of various organic halides on polymerization kinetics and on the oxidation state of vanadium derivatives has been examined in detail. Results suggest, as previously reported for lower temperature polymerizations, that the main role of the halogenated derivatives may be accounted for by their oxidizing character. It allows re-oxidation of the reduced V(II) species formed during the polymerization process, to valence III. The various pathways able to lead to the formation of new active species from (re-) formed V(III) are examined and discussed.