The valve covered by this specification is intended for use in connection with a pressure demand oxygen regulator, or any other regulator which will supply oxygen to the user in such a manner that, at altitudes above approximately 28,000 feet, a higher intrathoracic pressure than that of the ambient air is continually maintained. It is intended to be inserted into the expiratory chamber of an oxygen mask. A light check valve is incorporated within the valve. This check valve opens during inhalation, thus allowing the pressure in the inlet tubing and facepiece of the mask to equalize. Upon exhalation, the check valve closes, and the slight excess of pressure in the facepiece over the inlet line pressure causes the exhalation valve to open.