Recent toxicity studies on ionic liquids (ILs) are challenging their postulation as green solvents. Previous reports on mixtures containing ILs make it urgent to reveal the responsible components for the toxicity interactions. For that purpose, eight ILs, four consisting of 1-ethyl-3-methylimidazolium ([emim]) and the others of 1-butyl-3-methylimidazolium ([bmim], were selected as mixture components. The concentrations of eight ILs in mixtures were set up by the uniform design. The inhibition toxicities of single ILs and mixtures to Vibrio qinghaiensis sp.-Q67 were determined by microplate toxicity analysis. Combined toxicity was evaluated by the difference between the effects observed and predicted by the concentration addition model. Using the variable selection and modeling method based on the prediction (VSMP), it was found that the antagonism/synergism induced by the mixtures of eight ILs was related to [emim]BF4/[emim]CF3SO3. To further illustrate the toxicity interactions, eight ILs were split into two mixture groups, one containing four [emim]-based Its and the other four [bmim]-based ILs. The [emim]-group exhibited synergism while [bmim]-group resulted in antagonism. It was interesting that both the synergism and antagonism well related to the concentration ratio of ILs with BF4-. When ILs with BF4- were deleted from corresponding mixtures, the toxicity interactions (synergism/antagonism) disappeared. (C) 2012 Published by Elsevier B.V.