Abstract: In a method of storing energy in a two-vessel system, in which one vessel (1) containing a first substance (2) comprising two components, one of which is driven off from and is caused to become absorbed by the other component, respectively, at the supply of energy to and the withdrawal of energy from the system, respectively, energy is stored in the system by pumping said one component out of said one vessel (1).A system for carrying out this method comprises a two-vessel system, in which one chamber (1) contains a first substance (2) which comprises two components, one of which may be driven off from the other component, which on its side has the capability of absorbing said one component. The system also comprises an energy converter (6) adapted for the sucking or pumping out of said one component and which is incorporated in a conduit (4) connected to said one chamber (1).Thanks to the above method and system it has become possible to store mechanical or kinetic energy, e.g.