The effects of photolytic pretreatment on the biodegradation and detoxification of surfactants in anaerobic digestion were investigated by batch experiments with anaerobic digester sludges. Experimental results revealed that the surfactants were lowly biodegradable and highly toxic to methanogenic bacteria but a photolytic pretreatment with UV irradiation using a TiO2 catalyst was very effective at enhancing the biodegradability and reducing the toxicity of the surfactants. Both anionic sodium dodecyl sulphate (SDS) and sodium dodecyl benzene sulphonate (ABS), and nonionic p-nonylphenyl poly (oxyethylene) ether (NPE) brought about 50-60% reduction and cationic tetradecyldimethylbenzyl ammonium chloride (TBC) caused 100% reduction at a 100 mg/l concentration in the production of methane from glucose. With photolytic pretreatment of the surfactants under 4 h of UV irradiation, however, methanogenic activity was greatly enhanced in TBC and 30-50% increased in methane production in the other three surfactants as compared with unpretreated systems. The results imply that a combined system of photolytic and biological processes will be applicable to the treatment of wastewater containing toxic organics such as surfactants. It is also indicated that the photolysis of such aromatic surfactants as ABS, TBC and NPE is primarily a cleavage of the benzene ring and the degradation rate is expressed as a first order reaction.