A procedure for decomposition of contaminants in environmental water by simultaneous treatment with ozone and UV irradiation (ozone/UV treatment) was applied to linear dodecylbenzenesulfonate (DBS) in river water. Total organic carbon in the river water was decreased by this, though the reduction was not total and some persistent substances remained. When various river waters supplemented with DBS were treated with both ozone and UV, DBS was completely decomposed into inorganic carbons, while the decomposition was incomplete with ozone treatment alone. These results suggest that ozone/UV treatment is an effective procedure for purifying wastewater containing DBS.