A study was made of phase conversions and calculation of the thermodynamic functions of the system NH//4H//2PO//4-(NH//4)//2SO//4-H//2O AT 25 degree C. The water activity was measured by the isopiestic method in the binary systems (NH//4)//2-SO//4-H//2O and NH//4H//2PO//4-H//2O and in the ternary system NH//4H//2PO//4-(NH//4)//2SO//4-H//2O at 25 degree C. The binary and ternary Pitzer parameters were calculated from the data obtained. The solubility in the system NH//4H//2PO//4-(NH//4)//2SO//4-H//2O at 25 degree C was studied by the isothermal method. The experimental solubility data agree with the data calculated by Pitzer's equations.