A method of recovering hydrocarbons from a porous subterranean hydrocarbon-bearing formation by: (a) reducing the salinity of a saline source water by reverse osmosis using a membrane having a first surface and a second surface by (i) feeding the saline source water to the first surface of the membrane, and (ii) removing treated water of reduced salinity from the second surface of the membrane; and (b) injecting the treated water into the formation; wherein the membrane is selectively permeable to water over dissolved solids such that when (i) the saline source water has a total dissolved solids content of at least 17,500 ppm, and (ii) the applied pressure across the membrane is greater than the osmotic pressure across the membrane and lies within the range 45 to 90 bar (4.5 to 9.0 MPa), the total dissolved solids content of the treated water is in the range 500 to 5000 ppm. Increased oil recovery can be achieved.