3H-Perfluorobicyclo[2.2.0]hexan-2-one (3) has been synthesized from hexafluorobenzene and equilibrated with its enol form (4). In carbon tetrachloride K-e/k = 0.07 +/- 0.01 (25 degrees C), but in Lewis basic solvents (e.g. acetonitrile, ether, and tetrahydrofuran) only enol is detectable at equilibrium because of its strength as a hydrogen bond donor. In the monocyclic counterpart of this keto-enol system, 2H-perfluorocyclobutanone (1) and perfluorocyclobut-1-enol (2), the enol is more stable yet. Here ketone is undetectable under equilibrating conditions in all media examined, including carbon tetrachloride. Among unhindered and unconjugated enols, 2 and 4 are more stable relative to their ketones than any others that have been reported. Ab initio quantum mechanical calculations support the conclusion that destabilization of the ketones, but not stabilization of the enols, by fluorine substitution is responsible for the unique relative stability of these enols.