4-Diazoisothisochroman-3-one was found to undergo a photo-Wolff reaction upon flash photolysis in aqueous solution, generating 1,3-dihydrobenzo[c]thiophen-1-ylideneketene, whose hydration produced a second, readily observable short-lived species. The latter, through the shape of its decay rate profile plus solvent isotope effects, was identified as the enol of 1,3-dihdyrobenzo[c]thiophene-1-carboxylic acid. This behavior contrasts with that reported previously far the acyclic analogue of the presently studied system: flash photolysis of S-methyl phenyldiazothioacetate gives a photo-Wolff reaction as well, but the ketene so produced undergoes hydration 3 orders of magnitude more slowly than its cyclic counterpart; this makes ketene hydration in the acylic system Blower than enol ketonization, and that renders the acylic enol a low-concentration, nonobservable intermediate. Analysis of the rate profile for ketonization of the presently examined cyclic enol provides enol acidity constants as well as rate constants for ketonization of enol and enolate species.