In this paper, long-term operational data from a medium scale RO plant based on brackish water FilmTec spiral wound RO membranes are presented and analysed. It was found that over a period of 500 days of continuous operation, the water permeability coefficient for the overall plant decreased by about 25% and the salt rejection dropped by 1.9%. Simple formulae representing the long-term performance of the plant were developed. These expressions were found to fit the data with very good accuracy.