The effects of perfluorination of cyclic and acyclic alkenes have been examined. Quantum mechanical calculations and experimental data show that vicinally substituted acyclic alkenes are strongly destabilized by fluorination. Quite surprisingly, the destabilization is much smaller for cyclic analogs. This difference has dramatic consequences for the relative stabilities of double bond isomers in cyclic and acyclic systems, including keto-enol systems. The reasons for the contrast are discussed in detail.