The phenomenon of calcium sulphate deposits on reverse osmosis membranes was studied. A mathematical model was proposed and solved, which can predict the permeate flux, as well as the mass of deposits on the membrane as a function of time. The model is based on the following assumptions: (1) The rate of deposition is proportional to the deviations from equilibrium conditions. ( 2) The membrane permeability decrease is proportional to the mass of deposited salt. The ability of the model to describe the phenomenon and the accuracy of its predictions are satisfactory, as shown by the relevant experimental validation. LIST OF SYMBOLS A cb G C, J Jb Jf JO -Permeability coefficient of water per membrane area, m"/m" s atm -Salt concentration in brine, g/m3 -Salt concentration in feed, g/m3 -Permeate concentration, g/m3 -Permeate flux per membrane area, m"/m" s -Brine flux per membrane area, m3/m2 s -Feed flux per membrane area, m"/m" s -Initial permeate flux per membrane area, before the formation of deposits JT -Permeate flux per membrane area at steady state conditions, m"/m" s K -Constant, g/m3 (Eqn. 6) M -Constant, m"/g (Eqn. 4) m -Mass of deposits per membrane area, g/m2 *Author to whom all correspondence should be addressed. VR -Volume of reverse osmosis cell per membrane area, m"/m" AP -Difference between applied and osmotic pressure, kp/cm2