The time-resolved resonance Raman spectra and acid-base properties of the transient prepared on the submicrosecond time scale by N-3(.) oxidation of aqueous p-aminobenzoic acid (PABA) in near-neutral solutions identify it as the charge-neutral 4-(O2C)-O---aniline(.+) radical. This zwitterion intermediate undergoes slow thermal dissociation (k similar to 2.4 x 10(3) s(-1)) by intramolecular electron transfer and CO2 elimination. It reacts with common bases, such as OH-, N-3(-), and HPO42- ions, at the rate constants of 1.9 x 10(10), 2.7 x 10(6), and 2.2 x 10(8) M(-1) s(-1), respectively, and converts into the nondissociative anilino radical form (pK(a) 6.7). In the (OH)-O-. oxidation, formation of the zwitterion radical occurs via OH adducts (hydroxycyclohexadienyl radicals) of p-aminobenzoate anion, at a rate of 1.4 x 10(5) s(-1) which is independent of the base concentration. In strongly acidic solutions the OH adduct of p-aminobenzoic acid reacts with H+, at a rate constant of 4.7 x 10(8) M(-1) s(-1), to form the 4-HO2C-aniline cation radical (pK(a) 1.1). The benzidine cation radical, observed as one of the transient secondary products in Raman experiments, results from the reactions of p-aminophenyl radical produced on dissociation of the 4-(O2C)-O---aniline(.+) radical. The marked pH dependence of the nature and yields of the photoproducts of p-aminobenzoic acid, observed in several recent studies, is explained in terms of the dissociative properties of the zwitterion radical and its base-catalyzed conversion into the anilino radical. A two-state model has been developed for the explanation of the intramolecular electron transfer and bond dissociation in the radical intermediates with structurally stable ground electronic state.