The title compounds serve as potential precursors to aryloxenium ions, often proposed, but primarily uncharacterized intermediates in phenol oxidations. The uncatalyzed and acid-catalyzed decomposition of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, generates the quinol, 3a. O-18-Labeling studies performed in O-18-H2O, and monitored by LC/MS and C-13 NMR spectroscopy that can detect O-18-induced chemical shifts on 13C resonances, show that 3a was generated in both the uncatalyzed and acid-catalyzed reactions by C-alkyl-O bond cleavage consistent with formation of an aryloxenium ion. Trapping with N-3(-) and Br- confirms that both uncatalyzed and acid-catalyzed decompositions occur by rate-limiting ionization to form the 4-biphenylyloxenium ion, la. This ion has a shorter lifetime in H2O than the corresponding nitrenium ion, 7a (12 ns for 1a, 300 ns for 7a at 30 degrees C). Similar analyses of the product, 3b, of acid- and base-catalyzed decomposition of 4-acetoxy-4-methyl-2,5-cyclohexadienone, 2b, in O-18-H2O show that these reactions are ester hydrolyses that proceed by C-acyl-O bond cleavage processes not involving the p-tolyloxenium ion, 1b. Uncatalyzed decomposition of the more reactive 4-dichloroacetoxy-4-methyl-2,5-cyclohexadienone, 2b', is also an ester hydrolysis, but 2b' undergoes a kinetically second-order reaction with N-3(-) that generates an oxenium ion-like substitution product by an apparent S(N)2' mechanism. Estimates based on the lifetimes of 1a, 7a, and the p-tolylnitrenium ion, 7b, and the calculated relative stabilities of these ions toward hydration indicate that the aqueous solution lifetime of 1b is ca. 3-5 ps. Simple 4-alkyl substituted aryloxenium ions are apparently not stable enough in aqueous solution to be competitively trapped by nonsolvent nucleophiles.