Abstract: In this paper, the Pitzer equation (1973) and Pitzer-Li equation (1983) are used to correlate the activity coefficient data of di (2-ethyl-hexyl) phosphoric acid (D_2EHPA) in aqueous alkali chlorides (LiCl, NaCl, KCl, RbCl, CsCl and NH_4 Cl), which were determined by P~(32) radioisotope tracer technique. The calculated results show that the Pitzer-Li equation is better than the Pitzer equation, since the ion-solvent short range interaction energy and the ion-ion long range interaction energy are considered in the Pitzer-Li equation. The parameters thus regressed can reflect the sequence of the salting effect for alkali cation series and can explain the salting-out phenomenon for D_2EHPA. It also shows that the ion-water interaction energies are much greater than the ion-D_2EHPA interaction energies in th D_2EHPA-MCl-HCl-H_2O systems.