The influence of the temperature, the pressure, the Al/Ni molar ratio, and the position of chlorine-containing substituents in formazan ligands on the reaction kinetics and the product composition in the ethylene oligomerization catalyzed by thea system based on nickel(II) formazanates and ethylaluminium dichloride has been studied. It has been shown that effective metal complex catalyst systems can be prepared via varying the nature and the position of the substituents in the ligands.