Unstirred batch and cross flow nanofiltration have been carried out to separate dye from aqueous solution. The effect of different process parameters on separation of dye is studied. Experiments are conducted with aqueous solution of crystal violet (CV, molecular weight 408) using a 400 molecular weight cut-off membrane. An unsteady state mass transfer model is developed to predict the permeate flux and the permeate concentration in a batch cell. Using the experimental results, the model parameters i.e. the diffusivity of the solute (D) and real retention (R-r) of the membrane are evaluated by optimizing the experimental flux and permeate concentration profiles. Using the estimated D and R-r from the batch cell results, the permeate flux and concentrations are predicted for a cross-flow nanofiltration system using film theory. (C) 2003 Elsevier B.V. All rights reserved.