The influence of the co-ion nature on the nitrate-chloride selectivity coefficient in diluted/concentrated media has been studied. The variation of this selectivity coefficient as a function of the water activity of the solution was determined for three anion exchangers of various crosslinking degrees (X16, X4, and X2) in solutions of XCl-XNO(3) mixtures when X is K+, NH4+, Na+, and Li+. This study was performed under conditions in which the concentrations of both exchangeable anions (nitrate and chloride) in solution were similar and also when one of the exchangeable anions was predominant in solution. Results show that the nature of the co-ion can have an important influence on the nitrate-chloride selectivity coefficient and that this influence is higher when the resin crosslinking degree and solution concentration increase, that is, when the water activity of the solution decreases. The ratio of the concentrations in solution of both exchangeable anions may or may not modify the selectivity coefficient depending on the co-ion＇s nature.