Evaluation of the reverse osmosis (RO) process performance in an multistage/reverse osmosis (MSF/RO) hybrid system is of particular interest due to the obvious advantages of hybrid desalination plants over isolated ones. Meaningful evaluation of the RO process performance in a hybrid model requires continuous monitoring of an RO test unit for an appreciable period of time during the winter season when the ambient seawater temperature is low. The primary objective of this study was to systematically confirm possible gains in the RO product water flow rate and to examine the overall performance of an RO plant operating in a hybrid environment. Experiments were carried out at the Doha Desalination Research Plant (DRP) using a carefully designed and equipped 20 m(3)/d RO test unit linked with the nearby MSF unit A-1 of the Doha East Distillation Plant. Actual hybrid testing of the RO unit started during the cold season and continued for about 1,800 h. The temperature of the RO seawater feed withdrawn from the MSF reject stream ranged from 24 to 31 degrees C. Operating data collected from the RO test unit included temperature, pressure, flow rate, concentration, silt density index (SDI), and pH according to the function of each stream. A description of the experimental model, including the MSF/RO arrangement, performance data, and a discussion of the results obtained are included in the course of this paper. It was shown that an increase of up to 49% in the RO product water recovery was realized when data from before and after hybridization were compared. Also, a 42-48% gain in RO product water recovery was estimated for a seawater feed temperature of 33 degrees C as compared to an isolated RO plant using surface seawater at a temperature of 15 degrees C and at a reference feed pressure of 55 bar. Higher SDI values, increased frequency of backwash and chemical dosing, and loss of the membrane＇s salt rejection were also encountered during the hybrid testing.