The performance of a thin film composite spiral wound type membrane in reverse osmosis process is studied for rejecting the Cu+2 and Ni+2 from single-salt and mixed-salt systems. The removal ratios according to changes in basic parameters such as pressure and ion concentrations were measured and reported. The experimental results indicated that due to larger ionic size of Cu+2 its removal from the model solutions was more than Ni+2. Also the ion rejection in the different operating pressure was measured and reported. The obtained results indicated that the ion rejection depends on applied and osmosis pressures, however, by considering the operation limitations the ion rejection factor is R < 1. To examine the effect of increasing ionic size on the rejection by the membrane, the ions Cu+2 and Ni+2 were chelated by disoduim salt ethylene diamine tetraacetic acid (Na(2)EDTA) as a commonly used chelating agent. The obtained results indicate that Na(2)EDTA can increase the chelated ion size of Cu+2 and Ni+2 and thereby their rejection efficiency in the removal process by the membrane used.