A multicomponent mole-fraction-based thermodynamic model, together with Henry’s law constants and the vapor pressure of pure water, is used to represent aqueous phase activities, vapor pressures (of H2O, HNO3, HCl, and HBr), and saturation with respect to solid phases (ice, H2SO4 . nH(2)O, HNO3 . nH(2)O, and HCl . 3H(2)O) in the system HCl-HBr-HNO3-H2SO4-H2O. The model is valid from 328 to <200 K, and up to similar to 40 mol kg(-1) total solute molality for solutions containing mainly H2SO4 and HNO3. Model parameters for pure aqueous H2SO4 were adopted from a previous study, and values for HNO3-H2O, HCl-H2O, and HBr-H2O were obtained by fitting to activity and osmotic coefficients, electromotive force (emf) measurements, vapor pressures, freezing points, and thermal (enthalpy and heat capacity) data. The model was tested using measured partial pressures and solubilities of HCl in aqueous H2SO4 from >328 to 200 K, HBr solubilities in aqueous H2SO4 from similar to 240 to 205 K, and HNO3 partial pressures and freezing points for HNO3-H2SO4-H2O mixtures from 273.15 to <200 K. Ternary (mixture) parameters were required only for HNO3-H2SO4-H2O. Solubilities of HNO3, HCl, and HBr in liquid stratospheric aerosols are calculated.