Laser flash photolysis of the quinol ester 2b in O-2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O-2, but reacts rapidly with N-3(_) with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s(-1). Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N-3(-) completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N-3(-)-trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N-3(-)/solvent selectivity ratio, k (az)/k s, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by 02 or N-3(-). The lifetimes of the two intermediates of ca. 12 and 75 mu(s) are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (<= 20 ns) in Ar-saturated buffer. This species is suppressed, but not completely eliminated, in O-2-saturated buffer.