In this study, DS5 type nanofiltration membrane was tested to evaluate optimal operating conditions for membrane fouling and salt rejection in reactive dye and salt mixtures. Reactive black 5 (RB5) and NaCl were used in the experiments to prepare the synthetic dye and salt mixtures. Effects of pressure, cross flow velocity and pH were investigated. Experiments showed that cross flow velocities had a significant effect on flux values. In addition, effects of cross flow velocity decreased as NaCl concentration increased. At high pH values, low flux values were obtained due to the increase in dye hydrphobicity under alkaline conditions. Variations in pH, pressure and cross flow velocity also affected the salt rejection, which decreased with increasing NaCl concentration. Salt rejection at low cross flow velocities was lower than those conducted at high cross flow velocities and independent of the pressure, especially at low cross flow velocities and high pressures. (C) 2003 Elsevier B.V. All rights reserved.