The decomposition of p-hydroxybenzoic acid in aqueous solution by UV radiation and by H2O2 UV radiation has been studied. The experimental results indicated that the kinetics of both oxidation processes fit well by pseudo-first-order kinetics. In the second oxidation process, the overall kinetic rate constant was split into two components: direct oxidation by UV irradiation (photolysis) and oxidation by free radicals (mainly OH.) generated in the decomposition of H2O2. The effect of pH (2, 5, 7, and 9), temperature (10, 20, 30, and 40 degreesC), and initial hydrogen peroxide concentration (1, 2.5, and 5 mM) on the conversion of p-hydroxybenzoic acid was established. The importance of these two reaction paths for each specific value of pH, temperature, and initial hydrogen peroxide concentration was quantified. Last, a general expression is proposed for the reaction rate which takes into account both pathways and is a function of known operating variables.