The mean spherical approximation (MSA) has been applied to accurately correlate the activity coefficients of individual ions in aqueous solutions of single 1:1 electrolytes. The size parameters of the ions are considered to be composition-dependent in order to account for changes in the hydration of the ions. The values of the size parameters show that the hydration of the ions decreases as the electrolyte concentration increases. The dehydration of a particular ion depends on the nature of its counterion. The results obtained from the full MSA and from a simplified version of MSA (SMSA), are compared. The use of the simplified version has small effect on the final results and dramatically reduces the computational effort. The results obtained from the MSA compare favorably with those of the Bates' and the Fitter's models.