Why is the triplet state of aromatic ketones quenched by protons? The long-known but unexplained quenching process was investigated in detail for benzophenone (1). Adiabatic protonation of triplet benzophenone, 31, encounters a state symmetry-imposed barrier, because the electronic structure of (3)1 is n,pi*, while that of its conjugate acid, (3)1H(+), is pi,pi*. Hence, the rate of protonation of (3)1, k(H)+ = 6.8 x 10(8) M-1 s(-1), is well below the diffusion-controlled limit. The short-lived transient intermediate formed by protonation of (3)1 in 0.1-1 M aqueous HClO4 (lambda(max) = 320 and 500 nm, tau = 50 ns) is not (3)1H(+), as was assumed in previous studies. The latter (lambda(max) = 385 nm) is detectable only in acidified acetonitrile or in highly concentrated aqueous acid (>5 M HClO4), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of (3)1 is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, (3)1H(+) + H2O --> (3)1.H2O, k(0) = 1.5 x 10(9) s(-1). These findings lead to a revised value for the acidity constant of protonated (3)1, pK(a) ((3)1H(+)) = -0.4 +/- 0.1. Acetophenone (2) and several derivatives of 1 and 2 undergo a similar reaction sequence in aqueous acid. The acid-catalyzed photohydration of parent 1 and 2 is reversible. In meta-fluorinated derivatives, the reaction results in a clean and efficient formation of the corresponding phenols, a novel aromatic photosubstitution reaction. This indicates that hydration of (3)1H(+) occurs predominantly at the meta position. A long-lived transient (lambda(max) = 315 nm, tau = 5.4 s) left after the decay of (3)1.H2O is attributed to a small amount of ortho-1.H2O that regenerates 1 more slowly.