A combination of the classical Fenton reaction (Fe(II)+H2O2) with UV light, the photoassisted Fenton reaction, has been investigated for the treatment of landfill leachate. The investigation has been carried out with an experimental set-up to establish the optimal treatment conditions. The degradation rate of organic pollutants is strongly promoted by the photoassisted Fenton reaction. The degradation rate depends on the amount of H2O2 and Fe(II) added, pH value, and radiation intensity. At a specific energy input of 80 kW m−3 the oxidation rate was increased to six times the rate without radiation (0 kW m−3). At the higher radiation intensity of 160 kW m−3 the degradation rate was about two times faster than at that of 80 kW m−3. Due to the regeneration of the consumed Fe(II) ions through the irradiation, the amount of ferrous salt to be added can be remarkably reduced. The optimum conditions were obtained with 1.0 × 10−3 mol 1−1 Fe(II) added, a pH value of 3, and a molar ratio of COD: H2O2 = 1:1. At a COD volume loading of less than 0.6 kg m−3 h−1, a COD degradation of more than 70% could be obtained with an energy input of 80 kW m−3.