A cylinder valve for pressurized gas with a body including a gas inlet, a gas outlet and a passage connecting the inlet to the outlet. The valve also includes a closing/opening valve, a pressure reducer and a device for reducing or closing the gas passage between said valve and a pressure reducer. The device includes an element that is made of shape-memory alloy and positioned in a cavity of the passage. The element includes an active deformable portion, the circular outer edge of which is capable of engaging with the inner surface of the cavity in order to close the passage following a rise in temperature caused by combustion. The combustion can be generated by adiabatic compression during the opening of the valve.