Mean spherical approximation (MSA) model, is simplified, and used for the calculation of activity coefficients, for single 1: 1 electrolyte solutions. By using constant anion diameter, the cation diameters are calculated as a function of concentration. The cation diameters obtained are compared with those calculated by other models. The capability of the simplified MSA model to represent the activity coefficients of single aqueous 1: 1 electrolyte solution is demonstrated by application of the model to various kinds of single 1: 1 electrolytes. The calculated results have negligible error, and indicate the accuracy of the simplified MSA model. (C) 2004 Elsevier B.V. All rights reserved.