Recent advances in the theory of electrolytic solutions are used to describe osmotic and activity coefficients for aqueous solutions of electrolytes. The thermodynamic properties and also the structure are described by an extension of the mean spherical approximation (MSA) that includes association. The association of ions of opposite charges is described using the BIMSA, based on the Wertheim formalism. As was done before, hydration effects are included using a concentration-dependent cation size and permittivity. The theory is used to study aqueous solutions of mixed salts at 25 degrees C. The procedure yields a very satisfying representation for a number of mixed salts with a common cation where no adjustable cross parameter is needed. For salts without a common cation, cross parameters are introduced.