Osmotic coefficients of (NH4)(2)SO4(aq), and aqueous (NH4)(2)SO4-H2SO4 mixtures at 298.15 K and 323.15 K, and H2SO4(aq) at 323.15 K have been determined by an isopiestic method. Using the Fitter molality-based thermodynamic model, the measurements are combined with other literature data to obtain osmotic and activity coefficients of, first, (NH4)(2)SO4(aq) from <273 K to similar to 373 K and, second, aqueous (NH4)(2)SO4-H2SO4 over the entire composition range at 298.15 K.