Calculations at the B3PW91/6-311+G(2d,p) level of theory have been performed on a series of chlorofluoropropanes in order to account for the chemistry of the molecules CF2ClCF2CH3 and CF2ClCF2CD3, chemically activated in the gas phase, which form novel elimination products, CF3CF=CH2 or CF3CF=CD2, formally a 1,3-HCl or DCl elimination together with a 1,2-fluorine migration. The proposed mechanism involves an initial 1,2-FCl rearrangement, with an activation energy of 62.5 kcal/mol, giving CF3CFClCH3, which is 3.3 kcal/mol lower in energy than CF2ClCF2CH3. Subsequently CF3CFClCH3 eliminates HCl with a barrier height of 55.4 kcal/mol. This mechanism accounts for both the unimolecular kinetics and the small kinetic isotope effect. A concerted transition geometry has been characterized for the 1,2-FCl rearrangement of each molecule of the type CF2ClCXFCY3, where X and Y are H, D, or F; in each case the rearrangement leads to a more thermodynamically stable rearrangement product CF3CXClCY3.