Abstract: Water contamination of the oil in vacuum pumps of high vacuum systems is a major problem in maintaining efficient operation of those pumps. The problem is especially acute where a system includes an evacuated work chamber that must be repeatedly opened for loading products into and unloading them from that chamber where the ambient atmosphere has high humidity. The invention involves utilizing first stage mechanical vacuum pump means in conjunction with final stage high vacuum diffusion pump means, and a cryocoil with fast defrost capability located in the vacuum duct leading from the work chamber to the pumps, in combination with an auxiliary low capacity vacuum pump and a flip/flop valving arrangement which connects the discharge side of the diffusion pump selectively to the first stage mechanical pump or to the auxiliary pump.