Flash photolysis of 2-diazocyclohexane-1,3-dione in aqueous solution produced 2-ketocyclopentylideneketene, which hydrated to the enol of 2-oxocyclopentanecarboxylic acid, and the enol then isomerized to the keto form of the acid. This ketene proved to be a remarkably reactive substance, with an uncatalyzed hydration rate constant of k = 1.4 x 10(6) s(-1) and a hydroxide-ion-catalyzed rate constant of 7.5 x 10(7) M-1 s(-1); no acid catalysis of hydration was found. Ketonization of the enol was a slower process with rate constants in the millisecond to second range. The reaction shows a complex rate profile that could be interpreted in terms of rate-determining proton transfer to the beta-carbon atom of successively ionized forms of the enol in successively more basic solutions. In concentrated acid solutions, the carboxylic acid group of the enol also underwent equilibrium protonation of its carbonyl group, with pK(SH) -3.86. Acidity constants of the carboxylic acid group of the enol, pQ(a,E) = 4.16, and the hydroxyl group of the enol, pQ(a)(E) = 12.41, were also determined. (These acidity constants are concentration quotients applicable at an ionic strength of 0.10 M.) Rates of enolization of the keto form of the substrate were also measured by bromine scavenging, and these, in combination with ketonization rates, gave the keto-enol equilibrium constants pK(E) = 2.51 for the system in the un-ionized carboxylic acid form and pK'(E) = 3.00 for the ionized carboxylate form. The acidity constant of the carboxylic acid group of the keto form was determined as well: pQ(a,K) = 3.67.