This study was developed to highlight the influence of the nature of the water to be treated on the efficiency of nitrate elimination by a commercial membrane. By modifying the pH and gradually increasing the complexity of the treated solutions (binary, ternary and then natural waters), we identified phenomena which govern the ion transfer across the membrane. We underline the influence on membrane selectivity of the hydration energy as well as the membrane-solute and the solute-solute interactions. The observations made with simple nitrated solutions (binary and ternary) were extrapolated and confirmed in experiments made on polluted or nitrates-doped waters.