The retention of Na+ and Mg2+ ions in their nitrate solutions was studied in the pH range 0.5-7. Three different Mg2+ concentrations were used. Two nanofiltration membranes (NF 45 and Desal-5 DK) were tested. The NF 45 membrane showed higher retention values for the tested ions. The Mg2+ ions were very well retained (97.5-99.5%) within the pH range. The nitric acid permeated freely in all of the experiments. The experiments showed that the concentration of nitric acid in the permeate depended on the ratio of H+ ions/rejected metal ions in the feed. The retention of the Na+ ion varied from 80-40%. The smallest Na+ retention was measured in neutral conditions. The addition of HNO3 led to a sharp increase in the Na+ retention mostly because of two things: An increase in H+ concentration (a cation that permeates more easily than the Na+ ion) and the positive charge of the membrane due to the decrease in pH, increasing the rejection of all cations. In addition, the Na+ retention decreased with an increase of the Mg2+ concentration in the feed. The effect diminished gradually with decreasing pH due to a high ion concentration that lessened the charge effect of the membrane.