The mass production and wide usage of surfactants and aromatic compounds disturb seriously the natural ecosystem of physical

chemical and biological processes.The study on the mechanisms of joint toxicity can indicate the characterization of real environmental behavior and assessment of joint ecological effect.The toxicity to Photobacterium phosphoreum of different mixture systems of cationic surfactant of dodecyl dimethyl benzyl ammonium chloride (DDBAC) and three substituted aromatic compounds according to equi-concentration ratio of 1∶1

as well as individual toxicity

were determined respectively.They were evaluated by toxicity unit (TU)

additive index (AI)

similarity parameter (λ)

and mixtures toxicity index (MTI).Four evaluating methods were compared and the modes of joint action of different systems were analyzed.It was shown that coherent results were obtained by four different evaluating methods in most situations.Conclusions can be drawn that the joint effects of different mixture systems were synergistic.Mechanisms of joint toxicity action could be discussed according to luminescence principle of Photobacterium phosphoreum and molecule structures of the mixture components.