DIMERIZATIOI~ of n-butenes is of considerable practical interest for preparing xylenes and ethylbenzene [1]. Pure aluminium-alkyls both by themselves and mixed with intermediate metal compounds were used as catalysts for dimerization [1, 2]. Dimerization of n-butenes with triethylaluminium and nickel stearate requires strict reaction conditions (280 °, 172-185 atm, reaction time up to 6 hr). We have proposed a method [3] for dimerization of olefins under mild conditions [2] (20 °, normal pressure) by aluminium alkyl halides mixed with nickel salts. A description has been given in previous papers [4, 5] of dimerization o ° ethylene and propylene with these catalysts. The aim of this s tudy was to investigate dimerization of n-butenes. Whichever butene isomer is used, an equilibrium n-butene mixture, formed by rapid migration of the double bond, takes part in dimerization [4]. In accordance with earlier views on olefin dimerization under the effect of composite transition metal hydrides HMe, via formation of intermediate alkyl derivatives [5], it may be assumed that dimerization of n-butenes takes place according to this mechanism. The structure of the reaction products is confirmed by a study of dimer composition. In the I~rMR spectrum of the dimer (Fig. la) there are present proton signals of C H : C H groups (5.4 p.p.m, multiplet), ~C----CH-(5.2 p.p.m, multiplet), CH 2 in the R2C~CH 2 structure (4.7 p.p.m, multiplet), > C ~ C C H 2 (2.0 p.p.m, quintet), CH 3 in the R C H = C H C H a structure