Mutual diffusion coefficients of concentrated 1: 1 electrolyte solutions are investigated using a modified mean spherical approximation (MSA), in which the effective cationic diameter is a function of total ionic strength. We apply this improved MSA to calculate the equilibrium pair correlation function, and the Smoluchowski equation is used for the dynamics. We compare the calculated mutual diffusion coefficients of 18 uni-valence electrolyte solutions for molar concentrations up to 4.0 mol/L known experimental data. Good agreement between theoretical results and the experimental mutual diffusion coefficients are found for all the cases studied. (c) 2006 Elsevier B.V. All rights reserved.