A mole fraction-based multicomponent thermodynamic model, previously applied to the system H 2 O-H 2 SO 4 , is here extended to mixtures with (NH 4 ) 2 SO 4 at 298.15 K. Model parameters are determined by fitting to vapour pressure data (from isopiestic and electrodynamic balance studies), together with degrees of dissociation of the bisulphate ion and salt solubilities (solid phases (NH 4 ) 2 SO 4 , (NH 4 ) 3 H(SO 4 ) 2 and NH 4 HSO 4 ). The model successfully represents the thermodynamic properties of the system to high supersaturation over the full range of composition from (NH 4 ) 2 SO 4 -H 2 O to H 2 SO 4 -H 2 O. Equilibrium vapour pressures of NH 3 have been calculated for mixtures of various compositions.