A membrane based separation process (nanofiltration, NF) is used to treat the effluent from a textile plant. The dye mixture contains reactive black dye (Cibacron Black B) and reactive red dye (Cibacron Red RB). An organic membrane with molecular weight cut-off of 400 is used for the experiments. The experiments are conducted in an unstirred batch and a rectangular cross flow cell. Separations with retentions up to 94 and 92% of the two dyes are achieved respectively in the cross flow cell where steady state is attained quickly. It is important to note that NF techniques achieve a sharp reduction in chemical oxygen demand (COD), (up to 94% in cross flow cell), as the dyes are removed from the permeate. A parametric study of the separation process is undertaken to characterize the effects of the operating variables, e.g., trans-membrane pressure, dye concentration in the feed and cross flow velocity in case of cross flow NF. (C) 2002 Elsevier Science B.V. All rights reserved.