Abstract: An air admittance valve, particularly for use on a stack pipe of a toilet or drainage system, to close the pipe when there is at least atmospheric pressure in the pipe, but to open to admit atmospheric air to the pipe upon the occurrence of even a very small less-than-atmospheric pressure in the stack pipe. The valve has a hollow body (1,2) with a lower opening to the stack pipe, and an internal wall defining a chamber and a valve seat about an upward opening, and a lateral opening to the exterior. A valve member (3) in the body above the chamber opens and closes the upward opening, and the valve member is guided in alignment with the valve seat.