Abstract: For the investigation of the binary synergic extracting regularity, a multico-mponent system TBP-TTA-C₆H₆were studied. The activity coefficients of benzene and tributylphosphate (TBP) were measured by vapour pressure and ³²P-labeled TBPdistribution method respectively, then the activity coefficient of thenoyltri-fluroacetone (TTA) was obtained by Gibbs-Duhem equation. The activity coefficient of benzene, which dissolved two solutes-TBP and TTA simultaneously, have been also calculated by statistical theory using cluster expension method. The result is consistent with experimental data in the region of mole fraction of benzene greater than 0.6.