Ionic liquids (ILs) are novel green chemicals and used to replace traditional volatile organic solvents in industrial processes. Yet the potential effects of ILs on the toxicities of chemicals such as pesticides had been poorly studied. The aim of this paper is to determine the joint toxicity between IL and pesticide. Desmetryn (DES) and dichlorvos (DIC) were chosen as representatives of pesticides and 1-butyl-2,3-dimethylimidazolium chloride(IL1) and 1-butyl-pyridinium bromide(IL2)as those of ILs. The toxicities of the pesticides and ILs as well as their binary mixtures on Vibrio-qinghaiensis sp.-Q67 were determined using the microplate toxicity analysis. A simplified central composite design (SCCD) was employed to design the concentration distribution of components in binary mixtures to effectively detect the possible toxic interactions between pesticide and IL over the whole concentration range. Results showed that all the binary mixtures between pesticide and IL exhibited a similar toxicity action rule, i.e., displayed a synergistic interaction in a high concentration region. an additive action in a medium concentration region, and an antagonistic interaction in a low concentration region. The reason how to produce the toxic interaction is still under study. (C) 2009 Elsevier B.V. All rights reserved.