Flash photolysis of 2-diazocycloheptane-1,3-dione or 2,2-dimethyl-5,6,7,8-tetrahydrobenzo-4H-1,3-dioxin-4-one in aqueous solution produced 2-oxocyclohexylideneketene, which underwent hydration to the enol of 2-oxocyclohexanecarboxylic acid, and the enol then isomerized to the keto form of the acid. Isomerization of the enol to keto forms was also observed using solid enol, a substance heretofore commonly believed to be the keto acid. Rates of ketonization were measured in perchloric acid, sodium hydroxide, and buffer solutions, and a ketonization rate profile was constructed. Rates of enolization of the keto acid were also measured using bromine to scavenge the enol as it formed. Rates of enolization and ketonization were then combined to provide the keto-enol equilibrium constant pK(E) = 1.27. This and some of the other results obtained are different from the corresponding quantities for the 2-oxocyclopentanecarboxylic acid keto-enol system. These differences are discussed.