In this paper, factors influencing the flux decline of nanofiltration membranes for the treatment of dye bath wastewaters were investigated. Experiments were performed with synthetic wastewaters. Synthetic solutions were prepared in different dye and salt concentrations at laboratory conditions. Operating conditions including cross-flow velocity and pH were changed to observe the effects of interactions between NaCl, dyes and the membrane. Cake formation rate of dye molecules on membrane surface with time was investigated by using linearized forms of cake filtration equations. The results suggest that cake layer formation of dye molecules on membrane surface, especially at low salt concentrations was the principal cause of flux decline. Operating conditions had a strong influence on permeate flux. Effects of cross-flow velocity on permeate flux were more pronounced at low NaCl concentrations. Furthermore, the lowest permeate flux values were obtained at the alkaline conditions due to increased dye hydrophobicity at high pH values.