Carbon-acid ionization constants, Q(a)(K) (concentration quotient at ionic strength = 0.10 M), were determined by spectrophotometric titration in aqueous solution for benzo[b]-2,3-dihydrofuran-2-one (3, pQ(a)(K) = 11.87), benzo[b]-2,3-dihydrothiophene-2-one (2, pQ(a)(K) = 8.85), and benzo[b]-2,3-dihydrofuran-2-thione (1, pQ(a)(K) = 2.81). Rates of approach to keto-enol equilibrium were also measured for the latter two substrates in perchloric acid, sodium hydroxide, and buffer solutions, and the rate profiles constructed from these data gave the ionization constants of the enols ionizing as oxygen or sulfur acids pQ(a)(E) = 5.23 for 2 and pQ(a)(E) = 2.69 for 1. Combination of these acidity constants with the carbon-acid ionization constants according to the relationship Q(a)(K)/Q(a)(E) = K-E then gave the keto-enol equilibrium constants pK(E) = 3.62 for 2 and pK(E) = 0.12 for 1. The fourth, all-sulfur, member of this series, benzo[b]-2,3-dihydrothiophene-2-thione (4), proved to exist solely as the enol in aqueous solution, and only the enol ionization constant pQ(a)(E) = 3.44 could be determined for this substance; the limits pK(E) < 1.3 and pQ(a)(K) < 2.1, however, could be set. The unusually high acidities and enol contents of these substances are discussed, as are also the relative values of the ketonization and enolization rate constants measured; in the latter cases, Marcus rate theory is used to determine intrinsic kinetic reactivities, free of thermodynamic effects.