Nanofiltration (NF) (TFC-S) and reverse osmosis (RO) (TFC-HR) membranes were evaluated for the treatment of salty and polluted Kucukcekmece Lake used as a drinking water reservoir for the Istanbul City. A thin film composite type of spiral wound membrane (2 m(2) area) was used. Experiments were conducted at different pressures, pH ranges and temperatures. Flowrate was about 300 l/h. The flux decreased with increasing pH. Highest rejections were obtained for all anionic and cationic ions in the pH range of 6.0-8.5. Several permeation runs were carried out for the pressure range of 6-12 bar for TFC-S and TFC-HR membranes. According to the Spiegler-Kedem model, the permeate flux (Jv) is linearly related to the applied hydraulic pressure (DeltaP). L-p values were calculated by linear regression. Salt rejection was seen to increase with the increasing of operating pressure due to increase of solvent flux. Na+, Mg+2, Ca+2 Cl- and So(4)(-2) rejections and z(1) and z(2) constants have been determined and z values of cations were in the order of z(Mg) > Z(Ca) > Z(Na) and similarly those of anions were in the order of Z(SO4) > ZCl. The Spiegler-Kedem model was able to correlate well the experimental data by fitting different best values of P and sigma, respectively for all ions. Best fit values of P and a were obtained by regression of the data according to the model.