Background: A knowledge of the toxicity of ionic liquids (ILs) and their influence on aquatic ecosystems must be assessed before an accurate judgment of their environmental benefits and prior to their industrial application. In the present work, the overall octanol-water distribution coefficients (Dow) of imidazolium-based ILs were measured using the slow-stirring method. Biological tests were performed to establish the toxicity of some of these imidazolium-based ILs and to relate it with the Dow. Results: The Dow values of the ILs investigated are low and concentration dependent, indicating that these ILs, at low concentrations, will not accumulate or bioconcentrate in the environment. Moreover, it is shown that the most hydrophobic anions present higher Dow and higher toxicity, which means lower EC50 values (the concentration of IL needed to decrease the luminescence of the marine bacteria Vibrio fischeri by 50% compared with the control organism luminescence). Conclusions: A new and more accurate interpretation of the partition of ionic liquids between water and 1-octanol is presented. The work included measurement of new Dow data and the study of their relation with water solubility and EC50 values. © 2011 Society of Chemical Industry.