BACKGROUND: The breakdown of imidazolium ionic liquids (ILs) by Fenton oxidation has been demonstrated, but using unaffordable doses of hydrogen peroxide (H2O2). The literature studies so far do not report on the evolution of total organic carbon (TOC) and ecotoxicity. Therefore, the potential application of the process for the degradation of ILs still remains an open question. This work investigates the feasibility of the Fenton process for the treatment of aqueous effluents containing imidazolium-based ILs of different structures, represented by the length of the alkyl chain and the nature of the anion. Special attention is paid to the evolution of the ecotoxicity and the consumption of H2O2 as a critical issue for the economy of the process. The experiments were performed at 70 degrees C. RESULTS: With the stoichiometric H2O2 dose, the complete conversion of the starting imidazolium-based ILs was achieved in less than 5 min, at 70 degrees C, with 60% of TOC reduction after 4 h, except in the [C(4)mim][CH3CO2] case, where iron precipitation was observed, thus inhibiting its catalytic action. Low ecotoxicity effluents were obtained at the stoichiometric H2O2 dose, even in the cases of the highly ecotoxic ILs of long alkyl chain with chloride anion. CONCLUSION: Fenton oxidation is shown to be a promising solution for the degradation of imidazolium-based ILs in water, allowing deep detoxification with the stoichiometric H2O2 dose. (C) 2014 Society of Chemical Industry