The present invention concerns a semi-permeable membrane for use in osmosis consisting of one thin layer of a non-porous material (the diffusion skin), and one or more layers of a porous material (the porous layer), where the porous layer has a structure where porosity .phi., thickness of the porous layer (m), and tortuosity .tau., are related to one another as given by the expression: x.tau.=.phi.S wherein S is a structure parameter having a value equal to or less than 0.0015 meter. Further a method for providing elevated pressure by osmosis as well as a device for providing an elevated osmotic pressure and electric power is described.