Abstract Design and operating characteristics are analyzed for two pilot scale reverse osmosis plants. The first plant is installed in Sharjah to desalinate brackish water for a small community. The second plant is installed in Qatar to test the feasibility of desalting high-salinity seawater for irrigation purposes. Analysis is performed using a semi-empirical model, which requires knowledge of membrane salt reject and permeate recovery. Further analysis is made by using a permeability model, which is capable of predicting the required membrane area. Predictions of the two models are in good agreement with the available field data. Both models can provide the plant engineers with a useful tool to assess plant performance and analyze deviations from original design conditions.