Amnonium ion is effectively removed by Reverse Osmosis membranes (Brackish water, BW; Saline water, SW). The removal efficiency of ammonium ion from a solution containing ammonium ion which is not complexed is varying in the range of 5-60%. On the contrary, ammonium ion forms complexes with other ions in water and wastewater removal efficiency was as high as 99% for the synthetic solution which contained a complex of iron and ammonium ion. Elmali reservoir which is one of the water sources of the Istanbul city is suffering from high ammonia content due to discharges of untreated wastewater. Application of the BW membrane to Elmali Reservoir water resulted in ammonia removal efficiency of about 95%. The permeate ammonia concentration of 0.2 mg/l was achieved. High removal ratio of ammonia ions in complex form is due to increased molecular diameters of the ammonium ion complexes. Increased pH values were observed in parallel to increase the ammonium ion concentrations when ammonium ion was not in complex form. Ammonium ion removal efficiency decreased with increasing pH. On the other hand, when the concentration of ammonium ion complexed with iron was increased, pH value was decreased. In parallel to the decreasing pH, an increase of removal efficiency was observed. Membrane types did not significantly affect the flux. No fouling problems were observed during the test runs and thus fluxes were practically constant throughout the experimental run. The total estimated cost of treatment will be in the range of $0.95 to $1.06/m(3) for the investigated drinking water source.