1-Halovinyllithium carbenoids can undergo the Fritsch-Buttenberg-Wiechell (FBW) rearrangement to form alkynes. This reaction is quite facile when a hydrogen atom is present on carbon 2. The gas-phase reaction mechanism may consist of a single-step or several steps, depending on the aggregation state of the carbenoid. We examined the FBW reaction of 1-fluoro, chloro-, and bromovinyllithium in the monomer and two isomeric dimers. The migratory aptitude of the hydrogen atoms is dependent on the halogen and the aggregation state.