Mechanistic and kinetics details of 4-chlorophenol (4-CP) oxidation have been elucidated by radiolytic and photocatalytic techniques. When 4-chlorophenol is oxidized by gamma-radiolysis in conditions favoring hydroxyl radical oxidation ((OH)-O-.), significant concentrations of 4-chlorocatechol (4-CC) and hydroquinone (HQ) are formed as intermediates. Phenol is the only major intermediate when conditions favoring reduction by hydrated electrons are employed. 4-CC and HQ are not detected when 4-chlorophenol is oxidized with azide radicals. Hydroxyl radical mediated oxidative degradation rates of 4-CP are similar at pH 3.0 and pH 6.1. The 4-CP degradation rate is relatively slower at pH 9.1, and no aromatic intermediates are detected. These results confirm the proposal that hydroxy-mediated 4-CP oxidation follows at least three separate degradation pathways, forming 4-CC, HQ, and unidentified mostly nonaromatic compounds, as reaction intermediates enroute to complete mineralization. Pulse radiolysis experiments have also been carried out to characterize the radical transient species formed during the oxidation of 4-CP (pH 6 and 10) and 4-chlorocatechol (pH 6). The second-order rates for scavenging of (OH)-O-. radicals at pH 6 are measured as 9.3 x 10(9) M(-1) s(-1) for 4-CP and 7.0 x 10(9) M(-1) s(-1) for 4-CC. Photocatalytic degradation produces intermediates consistent with hydroxyl radical oxidation, but the concentration of aromatic intermediates is lower than in the radiolysis experiments, especially at higher concentrations of TiO2. This indicates that the course of photocatalytic transformation of 4-CP does not involve hydroxyl radical oxidation exclusively. Direct electron transfer and surface chemical reactions also contribute significantly to the disappearance of 4-CP and its reaction intermediates in TiO2 slurries.