A mole fraction-based multicomponent thermodynamic model, previously applied to the system H2O-H2SO4, is here extended to mixtures with (NH4)(2)SO4 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 (NH4)(2)SO4, (NH4)(3)H(SO4)(2) and NH4HSO4). The model successfully represents the thermodynamic properties of the system to high supersaturation over the full range of composition from (NH4)(2)SO4-H2O to H2SO4-H2O. Equilibrium vapour pressures of NH3 have been calculated for mixtures of various compositions.