For salinities covering the range from brackish water to highly saline sea water (50 fold range of salinities), effect of energy recovery (ER) on unit cost of desalination by reverse osmosis (RO) is sought. Since recovery is a primary factor in this respect, variables affecting recovery are considered in such a way that ensures trouble-free operation of RO units. Limiting conditions such as maximum and minimum allowable brine flow rates, maximum product flow, and limiting brine osmotic pressure are introduced as limiting factors in design calculations for one mgd (3785 m(3)/d) single stage plant. Results presented graphically show possible working ranges for different feed salinities. Maximum technically feasible recovery (Y-TF) defined for each salinity is determined and chosen for calculating cost-reduction expected from ER. A sensitivity analysis is also conducted to determine the effect of electrical energy unit cost. Results show that ER is expected to render cost reduction that warrants its use for seawater desalination at all normal levels of salinity. However, energy unit cost higher than 6 cents/kWh may be needed for noticeable cost reductions with brackish waters.