The long-term performance of a medium-scale industrial spiral wound reverse osmosis (RO) water desalination plant was studied. Operational data were analysed for a period of 1500 days as a basis for evaluating the performance variation with time. A theoretical model based on the solution-diffusion mass-transfer theory and concentration polarization was employed to extract the water and salt permeability coefficients. The results indicate that after over 4 years of continuous operation, the water permeability coefficient declined by about 39% and the average salt permeability coefficient increased by about 60%. Periodic membrane cleaning schemes were effective in partially restoring the water and salt permeabilities. (C) 2003 Published by Elsevier B.V.