Abstract: A check valve using a porous membrane to discriminate between gases and liquids. An elastomeric porous membrane disposed in the valve body at right angles to flow bears directly against a non-porous closure plate which is held off of a valve seat by a spring whose force acts opposite to the direction of fluid flow. The valve, therefore, is normally open. Air flows through the membrane with relatively little resistance and therefore passes through the valve unimpeded. Liquid, on the other hand, meets substantially high resistance to flow and creates a force on the membrane opposing the bias force exerted by the spring. Liquid, therefore, tends to displace the membrane and the closure member against the valve seat to close the valve. The device is advantageously applied to fuel management systems.