By considering the solution of the Mean Spherical Approximation (MSA) given by Lesser Blum, we addressed the problem of specificity in aqueous electrolyte solutions. The reference diameter of the cations is defined as the biggest possible diameter of the cation (obtained with the less associating anion). Then the specificity is taken into account by an association term similar to Bjerrum theory. The resulting activity coefficients can be used to describe pure electrolyte and electrolyte mixtures up to molar concentrations. Hydration appear to be relatively stable with small cations but it strongly depends on the anion for big cations. The validity of Zdanovskii-Stokes-Robinson mixing rule has also been tested and it appears to be valid with typically a 1% accuracy for molar concentrations.