The aim of this study was to estimate the biodegradability of the selected imidazolium ionic liquids and to determine the kinetic parameters for the biological treatment of wastewater containing these ionic liquids. Biodegradability was evaluated with the help of the Organisation for Economic Co-operation and Development (OECD) tests, while oxygen uptake rate (OUR) tests were made in order to calculate the values of Monod kinetic parameters. The results obtained from both types of the tests showed that ionic liquids of chemical structure of 1-alkyl-3-methyl imidazolium bromide were poorly biodegradable and co-biodegradable compounds, although their biodegradability increased with the elongation of the alkyl chain length. At the same time the presence of the imidazolium-based ionic liquids in wastewater at concentration of 50 mg l(-1) did not inhibit biomass growth as well as did not decrease the affinity of substrate to biomass. The values of both Monod kinetic parameters, i.e., maximum specific growth rate (mu(max)) and half saturation constant (K-s), increased with the increase in chain length of the alkyl substituent. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.