In the present study, the treatability of aqueous DEP solution employing H2O2/UV-C oxidation and the changes in acute and/or chronic toxicity of untreated and H2O2/UV-C treated DEP solutions was investigated. For DEP removal and its ultimate oxidation ( mineralization), an optimum initial H2O2 concentration of 40 mM was required. The HO center dot bimolecular reaction rate constant of DEP was found as 2.33 +/- 0.27 x 10(8) M-1 s(-1). Activated sludge inhibition experiments inferred that H2O2/UV-C treated DEP solutions were composed of a mixture of oxidation intermediates which exhibited higher degree of inhibition to heterotrophic biomass with respect to DEP. According to the results obtained in this work, the complete mineralization of DEP by H2O2/UV-C process would be a better option.