The salt effect of liquid-liquid equilibrium for the acetonitrile-water azeotropic mixture was studied at 25 ℃ and atmospheric pressure. Homogeneous acetonitrile-water mixture could be separated into two layers by the addition of salt such as NaCl, KCl, Na₂CO₃ and K₂CO₃. Particularly for the cases of NaCl, KCl and K₂CO₃ the upper layer composition shifted beyond the azeotropic composition. Mutual solubility and tie-line data were determined experimentally for ternary systems of acetonitrile-water-salt and the tie-line data were successfully correlated by using several known methods （Hirata, Campbell, Hand, Treybal） and modified plots (log x_s vs. log K_s, log y_s vs. log K_s). Plait points were also obtained from Hand-Treybal plots and modified plots. From the data of the distribution of salt and the selectivity of salt for water, it was found that the selectivity for the mixture was increased with increasing concentration of salt added and was maximum at its saturated state in the increasing order of Na₂CO₃, KCl, NaCl and K₂CO₃.