The mechanism of the (OH)-O-. oxidation of aromatic diamines in water has been examined using time-resolved Raman spectroscopy as a diagnostic tool and p-phenylenediamine (PDA) as a model system. It has been shown that the initial transient that is formed in the reaction in basic solutions is the cation (PDA(.+)) radical and not the (OH)-O-. adduct of p-phenylenediamine (PDA-OH.), as thought previously. In mildly acidic solutions, where PDA exists in its amine-protonated forms, a fraction of (OH)-O-. radicals is converted into the p-aminophenoxyl radical (APhO(.)) at the expense of PDA(.+). The production of PDA(.+) occurs in two steps: first, simultaneously with the (OH)-O-. adducts at.a diffusion-controlled rate and, second, on decay of the adduct radicals on microsecond times. Thus, transient absorption spectra display a marked pH dependence that was attributed previously to the acid-base conversion of the (OH)-O-. adduct and cation radicals. It is concluded that direct electron transfer (ET) is the dominant pathway in the (OH)-O-. oxidation of neutral diamines, which competes with adduct-mediated electron transfer (AMET) path on amine protonation.