Abstract The ultimate goal in the scientific study of desalting and water purification is to design a process that produces potable water at the lowest possible cost. Presently two major techniques are commercially used in different parts of the world: distillation and membrane processes. The dominant distillation process is multi-stage flash (MSF), and the main membrane process is reverse osmosis (RO). Although cost factors vary by site, the total cost of producing potable water from seawater with the RO process is usually less than thermal desalting processes. In this paper results of a study carried out in Kuwait to compare the unit cost of water produced from one-stage and two-stage hollow fiber (HF) and spiral wound (SW) seawater RO systems are presented. Results indicate that an average reduction in product water cost of about 22% could be achieved when one-stage RO is used instead of two-stage RO to produce drinking water from seawater in Kuwait. Moreover, an average of 11.84% of the capital investment cost can be saved by using a one-stage instead of a two-stage RO system. A greater reduction in product water cost is expected in the future as there is more room for improvement in the design, operation and maintenance of seawater desalination by RO technology.