The ethylene copolymerizations with 1-hexene or 1-octene in the presence of hydrogen using three catalysts, MgCl2(THF)(2)/VOCl3/Et2AlCl, MgCl2(THF)(2)/VCl4/Et2AlCl, MgCl2(THF)(2)/TiCl4/Et2AlCl, were investigated. It was found that the addition of hydrogen into the copolymerization feed reduces the molecular weight of the copolymers produced and decreases the activity of all the studied catalysts. The microstructure of the copolymers obtained was determined on the basis of C-13 NMR investigations and the reactivity ratios of the comonomers were calculated. The lack of tendency of the olefin comonomers to the creation of the polymer block was confirmed. It was found that the products of the comonomers reactivity ratios are close to the value of 1 and they are independent of both the catalyst and comonomer type. This means that the structure of all the copolymers obtained is random. (C) 2000 Elsevier Science Ltd. All rights reserved.