Acid ammonium sulphate and nitrate particles are major components of tropospheric aerosols. Aqueous concentrations within liquid aerosol particles are controlled by the ambient relative humidity, and substantial supersaturation with respect to dissolved salts is possible. The concentrated liquid particles that result at low relative humidity are, chemically, highly non-ideal. It is therefore difficult to predict size changes with relative humidity, and equilibrium with volatile components such as HCl, HNO 3 and NH 3 .Calculation of the activities of components of aqueous aerosols requires the use of semi-empirical thermodynamic models. Recently, we developed a new model for multicomponent solutions that is particularly suitable for applications involving aqueous aerosols (Clegg et al., J. Phys. Chem. 96, 9470-9479). It has been applied to the H 2 SO 4 -H 2 O system to < 1% relative humidity (Clegg and Brimblecombe, J. Chem. Eng. Data, in press), and to the mixture (NH 4 ) 2 SO 4 -H 2 SO 4 -H 2 O including supersaturated solutions (Clegg and Brimblecombe, J. Aerosol Sci., in press).Using new measurements, we have extended the chemical model to include components of the system H + -NH + 4 -NO - 3 -HSO - 4 -SO 2 - 4 -H 2 O, over a range of temperature. The model can be used to calculate equilibrium vapour pressures of water, HCl, HNO 3 and NH 3 , and saturation with respect to salts, and properties of supersaturated solutions, at different compositions.