The individual ion activity coefficient in an aqueous electrolyte solution is characterized in terms of two physical significant ionic parameters regarding the ionic solvation and the closest distance of approach between ions in a solution. The present approach was illustrated by the estimations of the individual activity coefficients of the ions of four electrolytes, NaCl, KCl, NaBr, and KBr in the aqueous single-electrolyte solutions. The estimation results showed the average absolute deviations (AAD) of the individual ion activity coefficients from the experimental values are <0.01 (except Na+ and K+ in KCl aqueous solution) for the aqueous single-electrolyte solutions. The vapor pressures of the aqueous two-electrolyte solutions consisted of the binary pairs of electrolytes of NaCl, KCl, NaBr, and KBr were estimated by the present approach and compared to the literature values. The average absolute relative deviations (AARD) of vapor pressures are <1.0% for these two-electrolyte aqueous solutions. The advantages of the present approach include: (1) only two parameters are involved for each constituent ion for the activity coefficient and the thermodynamic property estimations of the aqueous multi-electrolyte solutions. (2) Re-correlation from experimental data to determine new parameters is avoided when the additional electrolytes are involved since all the ionic parameters are simply calculated by a mixing rule. These two advantages make the present approach requires less computation effort than other methods do.