Abstract: The activity coefficients of TBPin various diluents have been determined by ³²P-labeled TBP distribution method. ³³P-labeled TBPhas been preparedby Higgin's method.It was dissolved in various diluents-benzene,hexane,carbon tetrachloride, chloroform and odourless kerosene. The composition of TBPsolutions could be varied with a wide range from 1 to 98 volume%. This method is simpler, more rapid and the range of cnocentration studied is broader than other physico-chemical methods.The radioactivities of aqueous phase equilibrated with ³²P-labeled TBPand its solutions were first measured, then the ratios A₂ of the radioactivities were calculated therefrom. The thermodynamic equilibrium constant λ⁰ is equal to the ratio of the activities of TBPbetween the two phases.As the solubility of TBPin water is low, the activity coefficient of TBPin water can be assumed to be equal to unity. Pure TBPis used as the standard state of solute for the determination of f₂ which is the activity coefficient of the solute TBP. Let The effect of different solvents on the activity coefficients thus measured has been discussed from the point of view of intermolecular interactions. As an alternate approach we choos ean infinitely dilute solution of TBPas the reference state so that the activity coefficients at infinitely dilute solution were taken to be unity. The standard state of the solute TBPwas obtained by extrapolating the Henry's law line to N₂→ and it is a hypothetical state.