Generation of the 4-biphenylyloxenium ion, 1a, from hydrolysis of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, is demonstrated by common ion rate depression and azide trapping. The ion is less selective with a shorter lifetime (12 ns at 30 degrees C) in aqueous solution than the corresponding nitrenium ion, 6a (ca 0.3 mus at 30 degrees C). Its lifetime is considerably longer than those of structurally related carbenium ions. Unlike 6a, 1a reacts with N3-, in part, at the distal ring to generate 4'-azido-4-hydroxybiphenyl, 5. These results are rationalized by calculations at the HF/6-31G* and pBP/DN*//HF/6-31G* levels that show that 1a is planar but is less stable than 6a relative to their hydration products by ca. 12 kcal/mol. The p-tolyloxenium ion, 1b, has not been unequivocally demonstrated to be formed in H2O, but kinetic data show that it must be at least 104-fold less stable than 1a relative to their respective 4-acetoxy derivatives. It is also calculated to be ca. 19 kcal/mol less stable than the corresponding nitrenium ion, 6b, relative to their hydration products.