BACKGROUND Ionic liquids are regarded as potential substitutes for volatile organic solvents. Thus, gaining knowledge about their biological properties is an important task. The main aim of this work was to estimate the biodegradability of newly synthesized tetra- and completely substituted imidazolium ionic liquids. Additionally, the kinetic parameters for the biological treatment of wastewater containing one of these ionic liquids were determined. RESULTS The results of OECD tests showed that none of the ionic liquids studied can be classified as readily or inherently biodegradable. The tetra-substituted ionic liquid with the longest alkyl side chain consisting of six carbon atoms was more susceptible to biodegradation than the other ionic liquids tested. The values of Monod kinetic parameters estimated for the biological treatment of wastewater containing one of these ionic liquids were usually in the range of the values determined for municipal wastewater. CONCLUSION Newly synthesized tetra- and completely substituted imidazolium ionic liquids are not susceptible to microbial decomposition. At the same time their presence in wastewater neither reduces the growth rate of activated sludge biomass nor decreases the affinity of substrate to biomass. (c) 2013 Society of Chemical Industry