The study presents the results of a new energy-efficient hybrid ion exchange-nanofiltration (HIX-NF) process to desalinate brackish and sea water. From a scientific viewpoint, the HIX-NF process is unique in its approach for it partially alters the feed water chemistry prior to the membrane treatment. Specifically it converts monovalent chloride, the predominant anion in salt water, into divalent sulfate through a reversible anion-exchange process without requiring any external regenerant. As a result, reverse osmosis (RO) membrane can be replaced altogether by nanofiltration (NF) membrane resulting in a marked reduction in energy requirement. Also, conversion of chloride to sulfate reduces the osmotic pressure of the feed water by nearly thirty percent and consequently decreases the theoretical energy requirement. The reject stream from nanofiltration, rich in sulfate, is used to regenerate the anion exchanger. Laboratory results validate that sodium sulfate can be desalinated with NF membranes at much lower transmembrane pressure than with RO membranes, all other conditions remaining identical. The reversible sulfate-chloride anion exchange and, in particular, sulfate/chloride selectivity plays a central role for the success of the proposed process. The most significant finding is that a single anion-exchange resin cannot sustain the HIX-NF process for different feed water concentrations. The investigation, however, shows that by changing the size of the amine functional group (e.g., quaternary-, tertiary-, secondary- and mixed amine), the sulfate/chloride selectivity of the anion-exchange resin can be tailored for a specific feed water salinity allowing optimum reversibility of the anion-exchange process. Laboratory studies have validated the basic premise of the hybrid process including greater than two times lesser energy requirement than RO process for the same feed water. Future proto-type experimental studies are strongly recommended to confirm the overall viability of the HIX-NF process. (C) 2008 Elsevier B.V. All rights reserved.