A multicomponent mole-fraction-based thermodynamic model of the H+-NH4+-Na+-SO42--NO3--Cl--H2O system is used to represent aqueous-phase activities, equilibrium partial pressures (of H2O, HNO3, HCl, and NH3), and saturation with respect to 19 solid phases (NH4)(2)SO4(cr), (NH4)(3)H(SO4)(2(cr)), NH4HSO4((cr)), NH4-NO3(cr), NH4Cl(cr), Na2SO4 . 10H(2)O((cr)), Na2SO4(cr), Na3H(SO4)(2(cr)), NaHSO4 . H2O(cr), NaHSO4(cr), NaH3(SO4)(2) . H2O(cr), NaNO3(cr), NaCl(cr), NH4HSO4 . NH4NO3(cr), (NH4)(2)SO4 . 2NH(4)NO(3(cr)), (NH4)(2)SO4 . 3NH(4)NO(3(cr)), (NH4)(2)SO4 . Na2SO4 . 4H(2)O((cr)), Na2SO4 . NaNO3 . H2O(cr), 2NaNO(3) . NH4NO3(cr). The model is valid for concentrations from infinite dilution to saturation (with respect to the solid phases) and to about 40 mol kg(-1) for acid sulfate systems which can remain liquid to concentrations approaching the pure acid. Parameters for H-2-SO4-H2O interactions were adopted from a previous study, and values for other binary (water-electrolyte) and ternary (water and three ions) interactions were determined from extensive literature data for salt solubilities, electromotive forces, osmotic coefficients, and vapor pressures. The model is compared with solubility measurements for the quaternary ion systems H+-Na+-SO42--Cl--H2O and NH4+-Na+-SO42--Cl--H2O.