.alpha.-Olefin dimer isomerization using organo-halogens

In a process for isomerization of a dimer of a lower .alpha.-olefin produced by dimerizing the lower .alpha.-olefin with a dimerization catalyst comprising a nickel compound and an organo-aluminum compound, the improvement comprising adding at least one compound selected from the group consisting of the following organo-halogen compounds of the formulae (I), (II), (III) and (IV), ##STR8## wherein X.sup.1, X.sup.2 and X.sup.3 are the same or different and are each a halogen or hydrogen atom or an alkyl group having 1 to 20 carbon atoms and at least one of them is a halogen atom, R is a hydrocarbon group having 1 to 20 carbon atoms and n is an integer of 0 to 5, ##STR9## wherein R.sup.1, R.sup.2 and R.sup.3 are the same of different and are each a hydrogen or halogen atom or a hydrocarbon group having 1 to 20 carbon atoms, X.sup.4, X.sup.5 and X.sup.6 are the same or different and are each a halogen or hydrogen atom or an alkyl group having 1 to 20 carbon atoms and at least one of them is a halogen atom,

• R.sup.4 --C.tbd.C--CX.sup.7 X.sup.8 X.sup.9 (III)
• wherein R.sup.4 is hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, X.sup.7, X.sup.8 and X.sup.9 are each a halogen or hydrogen atom or an alkyl group having 1 to 20 carbon atoms and at least one of them is a halogen atom, and ##STR10## wherein R.sup.5 and R.sup.6 are the same or different and are each alkyl group having 1 to 20 carbon atoms, or taken together, may form a cycloalkyl group having 3 to 7 carbon atoms, X.sup.10 and X.sup.11 are the same or different and are each a halogen or hydrogen atom or an alkyl group having 1 to 20 carbon atoms and at least one of them is a halogen atom to the reaction system, prior to deactivation of said dimerization catalyst, in an amount of 0.1 to 100 moles per 1 mole of said organo-aluminum compound in the catalyst, and then continuing the reaction at a temperature of -50.degree. to 200.degree. C. and thereby isomerizing said .alpha.-olefin dimer.