The theory of multilayer adsorption for concentrated aqueous electrolytes developed by Ally and Braunstein (1998) is used to calculate the thermodynamic activities and osmotic coefficients of supersaturated solutions of (NH4)(2)SO4 (aq) and NaCl (aq) which are among man-made aerosols of atmospheric importance. The basis of the model is the postulate of Stokes and Robinson that concentrated aqueous electrolytes may be viewed as an irregular ionic lattice structure. Application to the electrolytes mentioned above is a special case of the generalized results which are proven (Ally and Braunstein, Braunstein and Ally) to be consistent with the Gibbs-Duhem equation. Calculated thermodynamic properties are compared against experimental data from isopiestic and electrodynamic balance (EDB) measurements.