The gas-phase acidities and heats of formation of 2,4- and 2,5-cyclohexadien-l-one (1 and 2), the keto tautomers of phenol, have been measured by the flowing afterglow technique. The dienones were generated via pyrolytic [4 4+ 2] cycloreversion reactions of appropriate precursors. Bracketing experiments, involving reactions of 1 and 2 with a series of bases, established that ΔH°acid298(1) = 344 ± 3 kcal mol−1 and ΔH°acid298(2) = 3 40 ± 2 kcal mol−1. ΔH°f298(1) = -17 ± 3 kcal mol−1 and ΔH°f298(2) = -13 ± 3 kcal mol−1 were assigned by using the known acidity and heat of formation of phenol. The experiments establish unambiguously that the linearly conjugated dienone is more stable than its cross-conjugated isomer. Detailed comparison is made with the previously reported enthalpies of formation and aromatization of 1-methylene-2,4-cyclohexadiene and 1-methylene-2,5-cyclohexadiene (5 and 6), the triene tautomers of toluene. The tautomerizations of 1 and 2 to phenol are much less exothermic than the isomerizations of 5 and 6 to toluene, because the former processes entail endothermic enolization of a ketone. Phenol is significantly destabilized relative to toluene (ca. 11–13 kcal mol−1) by enolization. The resonance energies of 2 and 6 and of phenol and toluene are shown to be approximately equal.