A new high vacuum pumping system is described in which oil*is used in a diffusion type pump, and a trap containing activated cocoanut charcoal replaces the usual liquid air trap. The system is capable of attaining a pressure of 2×10−8mm Hg. A high degree of vacuum can be attained at least as quickly as with a mercury diffusion pump and liquid air trap. The system is especially adapted to maintaining a low pressure for several days in apparatus which cannot conveniently be sealed off from the pumps.