Abstract: A valve actuator system for use above or under water is disclosed wherein at least one gas generator (1, 2) is provided for controlling the system. The gas generator(s) can be actuated by remote control, by use of electric cables or hydro-acoustically without the use of cables. Locking mechanisms may be provided in order to lock the actuators in open and/or closed positions. The locking mechanisms may be actuated by the gas generator(s). Each gas generator is equipped with its own safety system, in that the charge itself is protected by a piston (26), thermal insulation (32) and a burst disc (31). The valve actuator is protected against excessive pressure by a safety shear disc (13, 14) connected with a dead volume (10). Two-way control of actuators is achieved by using a bisected piston system.