Abstract: Sea water is purified by reverse osmosis in an offshore installation wherein sea water is withdrawn from the sea by a low pressure pump and passed through plural-stage physical filters having decreasing degrees of porosity to remove suspended particles and colloids from the sea water. The thus filtered sea water is then fed by a high pressure pump, at a controlled pressure above the osmotic pressure of the sea water to a reverse osmosis module, wherein a portion of the sea water permeates through a semi-permeable membrane of the module to form purified potable water, and wherein the remaining sea water, having the dissolved salt concentration thereof increased, is discharged back to the sea. The reverse osmosis module is operated such that the conversion level, measured in percent and equal to the volume of purified water divided by the volume of sea water fed to the module, is maintained below a maximum value to prevent the formation on the semi-permeable membrane of the module of sulfates and carbonates.