Acetylketene was produced by flash photolysis of 2,2,6-trimethyl-4H-1,3-dioxin-4-one in aqueous solution, and rates of hydration of the ketene to acetoacetic acid enol and subsequent ketonization of the enol were measured in this solvent across the acidity range [H+] = 1 - 10(-13) M. Acetylketene proved to be a remarkably reactive substance, undergoing uncatalyzed hydration with the rate constant k = 1.5 x 10(6) s(-1), some 10(4) times more rapidly than ketene itself; the acetylketene hydration reaction was also catalyzed weakly by hydroxide ion but not by hydrogen ion. Ketonization of acetoacetic acid enol was much slower with rates in the millisecond to second range. The reaction showed a complex rate profile that could be interpreted in terms of rate-determining carbon protonaton of the carboxylate-ionized form of the enol in the acid region and rate-determining carbon protonation of the doubly ionized carboxylate-enolate form in the basic region. Analysis of the data provided the acidity constant pQ(a) = 4.05 for the carboxylic acid group of the enol and pQ(a)(E) = 13.18 for its enolic hydroxyl group. (These acidity constants are concentration quotients referring to an ionic strength of 0.10 M). Combination of the present results with information on the enolization of acetoacetic acid available from the literature gave K-E = 5.6 x 10(-3), pK(E) = 2.25, as an estimate of the keto-enol equilibrium constant.