In 1995 two reverse osmosis desalination plants, using hollow fiber membranes (types Dupont B-10) having 7000 m(3)/d and 800 m(3)/d production capacities respectively were set up at Laayoune and Boujdour cities. As to the design of this plant several parameters were taking into consideration including firstly the quality of raw water to be treated, local operating conditions, production capacities and expected costs. Since its commissioning, the operation of these plants has shown significant variability which was translated into a production extra cost specially for Boujdour desalination plant. By comparing the working conditions of the two plants it was possible to optimize their performances then to reduce the water production cost to normal values. Pretreatment, as the key for an optimum reverse osmosis plant working, was particularly concerned. The purpose of this paper is to highlight the scientific approach adopted in these desalination plants performances improvement.