The aim of this study was to investigate the effects of dyes, salts and auxiliary chemicals in reactive dye baths on the separation performance of nanofiltration membranes. A reactive dye bath was simulated for this purpose with auxiliary chemicals. A DS5-type nanofiltration membrane was used in the experimental runs. Performance of the nanofiltration membrane was evaluated by measuring permeate flux, salt and color rejections in five steps. Reactive black 5, reactive orange 16, NaCl, NaOH, Na2SO4, acidic acid, mollan and slipper were used to prepare synthetic dye baths. Pressures in the range of 8 to 24 bars were applied, and flow velocity was kept constant at 0.74 m/s. NaCl rejection of 20% and color rejection of more than 95% were achieved throughout the experiments. Permeate quality was satisfactory enough to recycle these effluents in reactive dyeing. Acidification of the original synthetic dye bath solution with HCl and H2SO4, decreased the membrane fouling and also increased the NaCl recovery and color rejection. Besides, using HCl instead of H2SO4 increased these positive effects. The effects of auxiliary chemicals were determined by using salt rejection model parameters of alpha and k(D) in the presence of an organic ion. There was a correlation among the results of experiments and the model. The model parameters (alpha and k(D)) were also calculated for all steps.