mercaptosuccinic acid etc in the thiol catalyzed isomerization of the dihalobutenes as well as in the anhydrous hydrogen bromide or chloride isomerization reaction it is necessary to employ an initiator for the reaction typical chemical initiators may be for example 2 2 39 azobisisobutyronitrile aibn benzoyl peroxide t butyl peroxide etc the amount of chemical initiator employed in the reaction is not critical but must be present in sufficient amount to initiate the reaction typically from about 0 1 mole to about 5 mole based on the weight of the dichlorobutene has been found to be effective as indicated above 2 mercaptoethanol as the catalyst and 2 2 39 azobisisobutyronitrile aibn as the initiator are preferred and have been found to be highly useful in the isomerization of dichlorobutene as they consistently provide ratios greater than 93 7 or higher trans cis dichlorobutene with 95 97 recovery of the dichlorobutenes hydrogen bromide with either aibn or ultraviolet light has also been found to be effective in producing remarkably high trans cis 95 5 ratios of dichlorobutene at room temperature hydrogen bromide is the preferred catalyst in the described reaction and has been found to be equally effective with either aibn or ultraviolet light.