The modified mean spherical approximation(MSA)model is used to predict the mean ionic activity coefficients for 32 aqueous mixed electrolyte solutions at 298.15K without any mixing parameters. The results show that the model gives a better representation of mixture properties in terms ofpure component parameters than Pitzers equation without mixing parameters. The mean standard deviation between the calculated and experimental values of 1n γ± is 0.0113. It is close to that of Pitzers equation with mixing parameters. The mean ionic activity coefficients in four mixed electrolyte solutions at saturated concentration and various temperatures have also been calculated by using the parameters obtained from single electrolyte solutions at 298.15K. The agreement between the calculated results and the experimental data is satisfactory over a wide range of temperatures. It shows that the present model has a powerfull capability for predicting the thermodynamic properties of aqueous mixed electrolyte solutions.