A valve (10) receives hydrogen under pressure, and is provided with an outlet (17) for conveying the gas at a reduced pressure to the anode (4) of a fuel-cell stack (5), and provided with a pilot member (28), which is movable for opening a valve seat (21) under the thrust of a first preloaded spring (35) against the action of the pressure of the gas at the outlet (17); a compensator (38) varies the pre-loading of the first spring (35) as a function of the pressure of the gas in a pre-defined point (39) between the outlet (17) and the anode (4); the compensator (38) has a membrane sensor member (60), which is movable in order to increase the aforesaid pre-loading under the thrust of a second spring (59) against the action of the pressure of the gas and is coupled to the first spring (35) via a mechanical lever transmission (80) .