A simple model is proposed within the framework of the mean spherical approximation (MSA) for a description of activity coefficients in aqueous electrolyte solutions. For simple salts, the model involves only one parameter per salt, which is the diameter of the hydrated cation. The optimum value of the latter is determined from a fit of the mean salt activity coefficient as a function of concentration in binary solutions of strong salts, up to an ionic strength of 3 mol kg −1. This model is the simplest that can be devised within the MSA. It is thermodynamically consistent for the ion activity coefficients. Its capability to predict activity coefficients in mixtures, without introducing any additional parameter, is examined by testing the results against experimental data for ternary solutions. The results are also compared with those from the (empirical) Davies equation, which is used in speciation programs. Finally, some predictions for the speciation of the self-complexing salt ZnCl 2 , in binary solutions and in mixtures with NaCl, are presented. They are compared with experimental data in the case of binary solutions. Such evaluations may provide relevant information for the speciation of toxic metal cations in brines.