Results of experimental studies on high-pressure ethylene polymerization in industrial autoclave and tubular reactors are considered using various peroxides and their mixtures with oxygen as initiators. A possibility for the process improvement providing the increase of conversion, productivity and versatility of the obtained polyethylene brands is demonstrated. According to the experimental data for the processes performed in single- and double-zone mixing mode reactors the initiator consumption, temperature profile stability and possibilities for rapid control over the process are analyzed depending on the type of the used peroxides or their complex mixtures. Peculiarities of ethylene polymerizations in tubular reactors using the initiators either individually (stepwise initiation) or in a complex (mixed) initiation mode are revealed and discussed. (c) 2004 Elsevier B.V. All rights reserved.