In this paper, we deal with the mean activity coefficient, gamma, of electrolyte solutions. The case gamma < or = 1 is investigated. As is generally recognized, the most accepted models (specific ion interaction/Pitzer theory) have the disadvantage of the dependence on semiempirical parameters. These are not directly accessible from experimental measurements, but can only be estimated by means of best-fitting numerical techniques from experimental data. In the general context of research devoted to the achievement of some reduction of complexity, we propose a model of electrolyte solution that allows us to calculate gamma without using fitting parameters where the (upper) concentration exists at which the electrolyte solution exhibits gamma = 1 (molality scale). In the remaining cases, we show that a unique parameter is required, that is, the concentration that should ideally give gamma = 1 for the electrolyte. Compared to other models that do not require adjustable parameters, the present one is generally applicable over a wider range of concentrations; moreover, it does not impose any restriction on the ion-size variations. Our model follows a pseudolattice approach, starting from the primitive idea of a disordered lattice of solute ions within a continuous solvent at extremely dilute solutions and coming to a disordered lattice of local arrangements of both solute ions and solvent dipoles at higher concentrations. Compared to other theories based on lattice models, this work stresses the role of statistical deviations from any time-averaged (lattice) configuration. All formulas in this paper are applied for 1:1, 2:2, 1:2, and 2:1 aqueous electrolytes at 25 degrees C.