Yields and distribution of the hydroxylation products of phenol in aqueous solutions were measured under the following conditions : (i) gamma-radiolysis, (ii) reaction with SO4.-, and (iii) photolysis in the presence of TiO2 colloid particles. Cupric ions as oxidants of OH adducts were present in all three systems. At relatively low phenol concentrations, the photochemical and the gamma-radiolysis systems show similar behavior, with the exception that the dependence of the product yield on pH is shifted toward lower pH’s in the photochemical system as compared to the radiolysis system. This is expected if the photolytic hydroxylation reaction takes place at the TiO2 surface where the positive charge of the TiO2 colloid particles induces a higher local pH compared with that in the bulk of the solution. At relatively high phenol concentrations, the photochemical system resembles the SO4.- system. This concentration effect results from the oxidation of adsorbed phenol by mobile TiO2 holes, which are the precursors of the localized holes. The mobile holes oxidize adsorbed phenol by electron transfer, while the latter react with phenol via an (OH)-O-. radical transfer. Our results indicate that localized TiO2 holes are relatively long-lived and are capable of reacting with nonadsorbed molecules.