An analysis of the impact of increasing RO membrane permeability on the reduction in water desalination cost was carried out for RO desalting operated up to the limit imposed by the thermodynamic restriction. The premise of the present assessment is that, the current generation of high permeability RO membranes makes it feasible to carry out RO desalination up to the thermodynamic restriction limit. In this limit, the ratio of membrane to energy cost can be expressed as a function of the water recovery level and a dimensionless cost parameter that accounts for the purchase cost of electrical energy and membrane area, as well as feed water salinity, salt rejection requirement and membrane permeability. The present analysis suggests that, given the present day electrical energy and membrane prices, the benefit of developing membranes of even greater permeability is primarily at low water recoveries for inland brackish water desalting. At low water recoveries, however, there is typically an added cost associated with brine management for inland water desalting. In RO desalting of seawater, on the other hand, the specific cost (i.e., per permeate produced) of energy is much higher relative to the membrane cost, and there is lower economic incentive for developing higher permeability membranes if the objective is to lower the cost of seawater desalination. The analysis suggests that further significant improvement in RO membrane permeability is less likely to be the primary driver for a major reduction in the cost of seawater desalination. However, it is expected that significant reduction in RO water production cost can arise from a variety of other process improvements including, but not limited to, development of improved fouling-resistant membranes, more effective and lower feed pretreatment and brine management costs, optimization of process configuration and control schemes, as well as utilization of low cost renewable energy sources. (C) 2009 Elsevier B.V. All rights reserved.