The unrestricted version of the mean spherical approximation has been applied to describe the concentration dependence of the mean ionic activity coefficients for 12 aqueous electrolyte solutions for which extensive permittivity data are also available. By using the actual solvent permittivity rather than that of the pure solvent, it is shown that this primitive model fits the experimental data in almost all cases to the maximum concentrations for which the permittivity can be described by the use of three adjustable parameters, namely, an effective ionic diameter, sigma(+) or sigma(-), the effective ion pair diameter sigma(ip), and the association constant K-as for ion pair formation. The values of K-as obtained are discussed with respect to results from experiments reported in the literature.