To achieve better management of processes and products, information on toxicity, safety, and risk assessment of chemicals is required. Ionic liquids are compounds of high interest for industry because of their attractive properties as solvents, but the water solubility of these compounds may lead to aquatic pollution and related risks. Experimental toxicity evaluation (Vibrio fischeri EC(50)) is a measurement of aquatic toxicity but there are theoretically over 1 million ionic liquids, which makes it necessary to estimate their properties by means of quantitative structure-activity relationships (QSARs). A database of Vibrio fischeri EC(50) was assembled to develop a novel group contribution method for estimating the EC(50) of ionic liquids. From the results the group contributions of anion, cation and alkyl substitutions has been calculated. The group contribution method allows an estimation of different combinations of groups in the toxicity (EC(50)) of ionic liquids.