Different studies were carried out to optimise the rejection of colour and sugar of black liquor using membranes. Highest rejection was obtained for 5000 molecular weight cutoff membrane (MWCO) for colour (97%) and sugar (80%). To obtained high throughput, the possibility Of using 100,000 MWCO membrane for the rejection of colour and Sugar was studied using variables Such as pH, ionic strength and additives. pH showed marginal increase in rejection, however, significant effect was observed on flux pattern of the membrane. Ammonium sulphate and polyethylene glycol showed significant effect on rejection as compared to polyacrylamide. Almost 90% rejection of colour and 68% rejection of sugar was obtained using 20% ammonium sulphate concentration. (C) 2003 Elsevier B.V. All rights reserved.