Mass transfer phenomena in crystallizing solutes and the concentrations of impurities in solution and in the crystal have been studied based on Maxwell-Stefan's theory of multi-component diffusion. The Maxwell-Stefan theory requires data on the driving force, i.e. the concentration gradient for each species. The effect of the electrolyte mixture on the diffusion coefficient of the crystallizing solute and the impurity solute was studied based on the Fitter theory, which can be used to determine the activity coefficients of a solute in electrolyte mixtures. The studied crystallization systems were the continuous suspension crystallization of potassium sulfate with sodium sulfate as an impurity, and the crystal growth study using potassium nitrate as a single crystal with calcium nitrate as an impurity in the solution. These two crystallization systems have different charge types of electrolytes. The proposed model was applied in calculating the mass transfer coefficient between the crystallizing solute and the impurity solute. Also, the molar flux of the specified species for the studied cases was estimated.