High-fluorine-content aluminum chlorofluoride, as prepared by Cl/F exchange of aluminum chloride with one of a number of organofluorine compounds, is a very active Lewis acid capable of condensing an allylic fluoride with another fluoroolefin at low temperature. In addition to a description of broader scope, details of the selective reaction of hexafluoropropene with tetrafluoroethylene to form F-pentene-2 are presented along with evidence supporting polyfluoroallyl cationic species as intermediates. Ab initio calculations confirm the feasibility of the proposed mechanism and further suggest that 1,3-fluorine shifts in fluorocarbocations are energetically accessible at modest temperatures. Revised heats of formation for C 3 F 8 (ΔH 0 f = - 1750 +/- 12.4 kJ mol - 1 ) and HFP (ΔH 0 f = - 1128 +/- 5 kJ mol - 1 ) have been calculated. Fluoride affinities of some simple metallohalogens are reported.