Reverse osmosis is widely used in Egypt for the production of drinking water from brackish and sea water. The heart of the RO system is the membrane that is capable of isolating dissolved salts from water through its wall under specific pressure. Simply, as much as the membrane wall is clean the productivity will be at its optimum condition, and vice versa. The present work aims to evaluate the pretreatment and operational practice that affects the performance and lifetime of the RO membranes in some small-scale running plants (less than 500 m(3)/d). These plants have different capacities and are used for the production of fresh water from either brackish or seawater. A field-survey was conducted to cover 7 desalting RO plants that show common problems in scale formation, and shortening of the membrane lifetime as well. In most cases the production rate declined down to less than 50% after a few months and the membrane life shortened to about one year. Two cases were presented in detail, the first case (brackish feed water) suffered from the deposition of calcium carbonate scale, while the second was completely blocked by massive crystalline gypsum as a result of desalting sea water. After problem identification, the probable causing factors were discussed regarding the misleading operational parameters and the types of chemicals used for maintenance and cleaning, It was found that all the investigated cases are using the "in-line" injection of chemicals without permitting adequate contact time for chemical reactions, The use of the polymerized antiscalants under this non-homogenized condition led to intermittent high concentrated waves that tend to agglomerate the scale potential chemical species rather than disperse it. It was also clear that the colloidal silica has strong affinity to polymerize scales of different compositions. The excessive concentration of bisulfite that is used for dechlorination seems to have a strong effect on scale formation too, On the other hand, the cleaning process that was described by the manufactures proved to be of mild action in such cases, and could be more aggressive. Some corrective actions had been applied successfully, however the proper controlling of silica was not satisfactory. Technical recommendations were proposed for better management of RO systems.