Abstract: A vacuum safety valve comprises a closed cylindrical valve body having one end connected to a system to be evacuated, and a tube extending from the side wall to a vacuum pumping system. The other end of the valve body includes a port connected to ambient air through a solenoid operated valve which is normally open when de-energized. An extendable bellows is secured about the port on the inside of the valve body, and the distal end of the bellows is sealed by a plate having a small bleed hole in the edge thereof. Should the pumping system fail, the solenoid valve is deactuated to open the bellows interior to ambient air. The air pressure expands the bellows and drives the plate against the interior port to seal off the system side of the valve before the vacuum therein is destroyed.