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 today usually less than thermal desalting processes and the economic advantages of RO vs. evaporation process, such as MSF, are responsible for the rapid increase in use of seawater RO. In many countries both RO and MSF processes are operating. Reduction in product water cost could be achieved when a hybrid MSF-RO system is used, instead of a parallel system to produce drinking water from seawater. An integrated system for the desalination of the seawater is proposed. The plant is composed of reverse osmosis and multistage flash in series. Electrical and thermal energy are supplied by a cogeneration group for the production of electrical and thermal energy connected to the public net. From a first analysis, the proposed plant seems to allow some simplifications and cost reduction for the fresh water, compared to the equivalent systems in parallel.