There are many ways to improve efficiency and reduce fresh water cost in desalination plants, like the integration of two or more desalination systems into a hybrid mechanical/thermal lay-out. Most common results that could be achieved by hybrid systems are savings in pretreatments, overall cost reduction and significant primary energy savings, especially when cogeneration unit are concerned. In this paper a small size (2000 m(3)/day) thermal desalination system (MEE) is coupled with a single-stage seawater reverse osmosis (SWRO) system; the system is fed by a natural gas (NG) reciprocate engine, where heat is recovered both from exhaust gases and from the cooling jacket water circuit. Technical and energetic aspects of the examined hybrid lay-out are described in detail, and complemented with a brief cost analysis in order to assess the economic viability of the proposed solution. In the examined case, cogeneration coupled with hybrid desalination systems reveals economically convenient with respect to the production of the same fresh water flow rate by very efficient RO systems. The minimization of the unit cost of desalted water was the goal of the performed optimization; the sensitivity of fresh water unit cost to variations in the flow rate of permeate exiting the RO section is also investigated to determine the optimal design.