Rate ratios were obtained for the base-catalyzed 1,4-dehydrochlorination of (E)- and (Z)-Me3CCCl:C:C:CClCMe3 to give Me3CC.tplbond.CC.tplbond.CCMe3. In the KOMe/MeOH or KOCMe3/Me2SO systems the reactivity of both isomers was similar. With KOCMe3 in Me3COH, THF or C6H6 the 2 isomers exhibited similar reactivity when 18-crown-6 was present, but the E isomer was much more reactive in its absence. These findings suggested a conveyor-belt-type transition state with an aggregate of the base interacting with the H and Cl atoms undergoing elimination. [on SciFinder (R)]