Abstract: The dehalogenations of 1, 5-dihalobicyclo [3.1. O] hexanes and 1, 6-dihalobicyclo [4.1. O] heptanes have been studied in solution and in the gas phase. The solution reactions led to the formation of bicycl0 [3. lO] hex-l (5)-ene and bicyclo-[4. I. O] hept-l (6)-ene, respectively, but this was followed by rapid ene reactions forming dimers, which then coupled to form tetramers. The cyclopropenes could be trapped as Diels-Alder adducts. In ...