A new correlation predicting the activity coefficients of aqueous electrolyte solutions between Pitzer ion interaction parameters and ionc properties (ionic charge, radius and entropy) is developed. The substances include uni-uni and uni-bi type of single salts for which data are available. The results of correlation show linear relations for uni-uni type, and quadratic equations for uni-bi type. Many of the trends in the calculated activity coefficients for electrolyte solutions can be correlated with respect to the effect of the ions on the structure of the water and explained by structure-making and -breaking concept. These kinds of correlation may be useful in predicting values for salts that have no measurement of the activity coefficients.