There are often many chemicals coexisting in aquatic ecosystems, and information the joint toxicity of a mixture of organic pollutants on microorganisms is scarce at present. Acute toxicity of aromatic anilines and phenols and their mixtures to alga was determined by the algae inhibition test. The median effective inhibition concentration EC(50) values for single compounds and EC(50mix) values for binary and multiple mixtures were obtained. The joint toxic effects of mixtures were estimated by using mixture toxicity index method. The structural descriptors of the n-octanol/water partition coefficient (log P (mix)) and the frontier orbital energy gap (DeltaE (mix)) for mixtures were calculated. Based on the quantitative structure-activity relationship model for single chemical toxicity log(1/EC(50)) = 0.579log P - 0.783DeltaE + 8.966 (n = 11, r (2) = 0.923), the following two-descriptor model was developed for the toxicity of a mixture: log(1/EC(50mix)) = 0.416log P (mix) - 0.584DeltaE (mix) + 7.530 (n = 27, r (2) = 0.944). This model can be used successfully to predict the toxicity of a mixture, whether binary mixtures in variant toxic ratios (4:1, 2:1, 1:1, 1:2 and 1:4) or multiple mixtures of three or four chemicals at an equitoxic ratio are used as predictors.