The invention pertains to a method and a device for desalting water by reverse osmosis, particularly for desalting seawater, in which saltwater is introduced under a first pressure into a pressure-compensation device and conducted from pressure-compensation device at a second, higher pressure into a membrane module, wherein desalted water and concentrated saltwater are conducted out of membrane module, wherein the concentrated saltwater conducted out of membrane module is continuously introduced at roughly second pressure into pressure-compensation device and used there to apply roughly second pressure to saltwater introduced into pressure-compensation device and to introduce saltwater into membrane module, and wherein the introduction of concentrated saltwater into pressure-compensation device and the conduction of concentrated saltwater out of pressure-compensation device are accomplished by means of controlled main valves. The main valves are subject to high mechanical stresses, particularly during the opening and closing of the valves. To reduce or avoid these, it is provided according to the invention that secondary valves arranged in parallel with main valves are controlled such that load peaks during opening and/or closing of main valves are reduced.