Alkylbenzenesulfonates have been used mainly as synthetic detergents, and their decomposition methods by ozone and UV radiation were investigated in this paper. Standard substance of sodium dodecylbenzenesulfonate (LAS) was used and the aqueous solutions were treated with ozone (1.5 mg O₃/60 cm³/min). UV radiation (15 W low pressure mercury lump) was applied from the interior of the solution. LAS (1×10^-3M) was treated with ozone + UV radiation after pretreatment with UV radiation for 3 h. When the removals of LAS were determined by the HPLC method and ethyl violet method, anionic surfactants were decreased by UV radiation alone, but TOC removal was almost negligible. Then TOC was decreased by simultaneous ozone and UV treatment. Decomposition of LAS to acidic compounds was suggested from the time courses of surface tension, pH and the absorbance at 225 nm. LAS (1×10^-4M) was treated by ozone + UV radiation without pretreatment, and decomposed rapidly. TOC was decreased by ozone + UV radiation at pH 7.0 more rapidly than at pH of 2.0 or 10.0. Therefore the decomposition method by ozone + UV radiation at neutral pH will be valuable for effluents containing LAS.