A thermodynamic model of the system H+-NH4+-Na+-SO42--NO3--Cl--H2O is parametrized and used to represent activity coefficients, equilibrium partial pressures of H2O, HNO3, HCl, H2SO4, and NH3, and saturation with respect to 26 solid phases (NaCl(s), NaCl center dot 2H(2)O(s), Na2SO4(s), Na2SO4 center dot 10H(2)O(s), NaNO3 center dot Na2SO4 center dot Na3H(SO4)(2)(s), NaHSO4(s), NaHSO4 center dot H2O(s), NaNH4SO4 center dot 2H(2)O(s), NaNO3(s), NH4Cl(s), NH4NO3(s), (NH4)(2)SO4(s), (NH4)(3)H(SO4)(2)(s), NH4HSO4(s), (NH4)(2)SO4 center dot 2NH(4)NO(3)(s), (NH4)(2)SO4.3NH(4)NO(3)(s), H2SO4 center dot H2O(s), H2SO4 center dot 2H(2)O(s), H2SO4 center dot 3H(2)O(s), H2SO4 center dot 4H(2)O(s), H2SO4 center dot 6.5H(2)O(s), HNO3 center dot H2O(s), HNO3 center dot 2H(2)O(s), HNO3 center dot 3H(2)O(s), and HCl center dot 3H(2)O(s)). The enthalpy of formation of the complex salts NaNH4SO4 center dot 2H(2)O(s) and Na2SO4 center dot NaNO3 center dot H2O(s) is calculated. The model is valid for temperatures less than or similar to 263.15 up to 330 K and concentrations from infinite dilution to saturation with respect to the solid phases. For H2SO4-H2O solutions the degree of dissociation of the HSO4- ion is represented near the experimental uncertainty over wide temperature and concentration ranges. The parametrization of the model for the subsystems H+-NH4+-NO3--SO42--H2O and H+-NO3--SO42--Cl--H2O relies on previous studies (Clegg, S. L. et al. J. Phys. Chem. A 1998, 102, 2137-2154; Cars law, K. S. et al. J. Phys. Chem. 1995, 99, 11557-11574), which are only partly adjusted to new data. For these systems the model is applicable to temperatures below 200 K, dependent upon liquid-phase composition, and for the former system also to supersaturated solutions. Values for the model parameters are determined from literature data for the vapor pressure, osmotic coefficient, emf, degree of dissociation of HSO4-, and the dissociation constant of NH3 as well as measurements of calorimetric properties of aqueous solutions like enthalpy of dilution, enthalpy of solution, enthalpy of mixing, and heat capacity. The high accuracy of the model is demonstrated by comparisons with experimentally determined mean activity coefficients of HCl in HCl-Na2SO4-H2O solutions, solubility measurements for the quaternary systems H+-Na+-Cl--SO42--H2O, Na+-NH4+-Cl--SO42--H2O, and Na+-NH4+-NO3--SO42--H2O as well as vapor pressure measurements of HNO3, HCl, H2SO4, and NH3.