The Tremiti desalination plant assures the drinkable water supply to the Island of S. Nicola in the Adriatic Sea. The system is a stand-alone plant for sea water desalination using the reverse osmosis process, energized by a photovoltaic power plant. A p.v. array gives a 65 kWp nominal peak power. DC/DC converters feed the main d.c. bus and the storage battery which assures the uninterrupted operation of the desalination plant during overcast periods. The total plant area is 5000 sq. m; 1800 of them covered by the p.v. field: the layout of the rows of modules has been designed to optimize the efficiency of the array subsystem, to minimize atmospheric overvol-tage, and to assure easy operation and maintenance. Good fitting with the natural characteristics of land is obtained. The final solution of the supporting structure satisfies to the last Italian codes for seismic areas: pv modules are bolted to galvanized steel beams supported by walls of vibrated concrete. The electric appliance load consists of a.c. induction motors driven by single inverters. The R.O. plant produces about 5000 cu. m. of water with an average power requirement of 13 kWh/cu. m. A power flow supervisory system optimizes, in every possible operating condition and system status, the utilization of available energy. A data acquisition system monitors continuously the system and records the relevant parameters.