Abstract: Water is purified by reverse osmosis using a semi-permeable membrane supported by the outer surface of an extruded tube of water-permeable rigid p.v.c. so that impure water is recovered from the bore of the tube, the p.v.c. having an apparent density of from 0.5 to 1.3 g/cc, and an average pore diameter of from 5 to 500 microns, at least 50% of the pores being interconnected and 95 to 99.5% of the pores in the tube being accessible from the inner or outer surface thereof, the tube having 1 to 10 holes/mm.sup.2 on the outer surface thereof, and having no surface holes larger than 400 microns in diameter and external diameter of from 1.5mm to 8mm and having a wall thickness of at least 0.5mm.