The influence of counter ions (Cl-, C(CN)(3)(-) and CF3SO3-) and background ions (C6F11O2-, C8F15O2- and C10F19O2-) on the degradation rates of 1-butyl-3-methylimidazolium cation (bmim(+)) in a Fenton-like system were investigated. The degradation rate of bmim(+) was influenced by the particular anions as follows: Cl- > C(CN)(3)(-) > CF3SO3-. The inhibition of imidazolium moiety oxidation was observed in the case of counteranions in ionic liquid which entered the competition for hydroxyl radicals with bmim(+) which formed complexes with ferric and ferrous ions. The presence of perfluoric acids as a background in solution also had significance for the bmim(+) degradation process. These organic anions can inhibit bmim(+) oxidation through strong interactions between positively charged bmim(+) and negatively charged perfluocarboxylic anions (PFCA). Additionally, bmim(+) seemed to be more resistant to oxidation in the presence of a longer n-perfluoric alkyl chain anions. However, it was observed that the inhibition of bmim+ oxidation decreased when the concentration of PFCA in solution exceeded a critical micellar concentration. This fact can be explained by weaker interactions between bmim(+) and perfluoric anions, stronger associated with aggregated PFCA in these experimental conditions (pH 3). Our investigations also indicated the possibility of PFCA interactions with ferric and ferrous ions. (C) 2009 Elsevier B.V. All rights reserved.