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This Accepted Manuscript will be available for reuse under a CC BY-NC-ND license after 24 months of embargo period ; The catalytic wet peroxide oxidation (CWPO) of the imidazolium-based ionic liquids 1-butyl-3-methylimidazolium chloride (BmimCl), 1-butyl-3-methylimidazolium acetate (BmimAc), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BmimNTf2), 1-hexyl-3-methylimidazolium chloride (HmimCl) and 1-decyl-3-methylimidazolium chloride (DmimCl) was examined by using a Fe catalyst supported on alumina (Fe2O3/Al2O3) that was prepared by incipient wetness impregnation. Variable H2O2 doses from 0.5 to 1.5 times the stoichiometric value provided similar results in terms mg TOC removed per mg H2O2 decomposed at 80 °C (0.033 mgTOC mgH2O2−1), all allowing complete Bmim+ removal. Raising the reaction temperature to 90 °C increased the mineralization rate up to 40% TOC conversion. Differences in TOC conversion among counteranions (chloride, acetate and NTf2−) were negligible. A plausible reaction pathway is propose involving hydroxylated compounds and short-chain organic acids as reaction byproducts. CWPO markedly increased the subsequent biodegradability of the IL test solutions and led there to TOC conversions after CWPO-biodegradability assays of 55–60%. The Fe2O3/Al2O3 catalyst exhibited high long-term stability; thus, it retained most of its properties and underwent negligible Fe leaching. ; The authors acknowledge funding from Spain's MINECO (CTM2016-76564-R), the Madrid Regional Government (S2013/MAE-2716), UAM-Santander (CEAL-AL/2015-08) and UNAM Engineering Institute (II-4307). I. F. Mena also thanks MINECO and ESF for award of a research grant