The trouble-free operation of a Reverse Osmosis (RO) plant relies on the quality of the treated feed water, which highlights the need for optimisation of the pre-treatment steps. The quality of raw seawater varies with time in terms of suspended and colloidal load, along with the dissolved salt content. In a conventional pre-treatment scheme, consisting of clarification and particulate and micron filtrations, along with chemical conditioning, the chemical dose rates are required to be pre-determined and monitored according to the changing raw water quality. In the RO section, operating pressure and membrane area should be variables as the need arises, depending on the variation in dissolved salts, for maximum production of better quality water. In this paper, the theoretical evaluation of RO performance with respect to seasonal variation in feed salinity is presented, which tells of the need to incorporate sufficient flexibility in operation when designing a plant. Copyright © 2006 Inderscience Enterprises Ltd.