In the present work a set of reverse osmosis experiments with operating pressures up to 200 bar and sodium chloride concentration up to 200 g/l were carried out, in which a pilot plant with two disc-tube-modules (a special type of plate and frame modules) was used. Each had an membrane area of 7.66 m 2 . The influence of high pressures and concentrations on mass transfer through the membrane and within the boundary layer will be described in the following.In the second part of this paper some aspects with respect to engineering, plant design and economics of high pressure reverse osmosis systems will be discussed.