This study is focused on the influence of the nature of the water to be treated on the efficiency of nitrate elimination by a commercial membrane. By gradually increasing the complexity of the treated solutions (binary, ternary mixtures and then natural waters), we identified the physical, chemical and electric phenomena, which govern the ion transfer. We highlighted the influence on the membrane selectivity of the hydration energy of the solute, of the membrane-solute and solute-solute interactions. The observations made with simple nitrated solutions (binary and ternary) were extrapolated and confirmed in experiments on the water doped out of nitrates.