A batch reactor was used to evaluate the efficiency of advanced oxidation process of the organic pollutants in biologically treated wastewater effluents with UV/H2O2. A 450-W high-pressure mercury vapor lamp was used as the light source. During the degradation process, the concentration of the dissolved organic compounds could be increased by more than twofold due to the decomposition of microorganisms. This increase of the dissolved organic compounds was eliminated if the water was filtered before the photodegradation experiments. It is observed that the UV alone could play a role for the oxidation of the organic pollutants; however, the addition of a small amount of hydrogen peroxide promotes the degradation efficiency of organic compounds in wastewater. The best oxidation efficiency was obtained when the water samples were under acidic conditions (pH 5), and the rate of degradation was not enhanced with the increasing H2O2 dosages. The optimum H2O2 dose was between 0.01% and 0.1% for the oxidation processes in this study. The presence of the carbonate/bicarbonate ions in water inhibits the degradation of the organic compounds.