A vacuum pump system (10; 36; 46) for pumping gas from an enclosure (12) comprises: a vacuum pump unit (14) having a pump inlet (16) connectable with an outlet (18) of such an enclosure and a pump outlet (20) for exhausting gas; and a chamber (22; 50) selectively connectable with the pump inlet and the pump outlet, such that, in use, in a first state gas can be pumped from the chamber to the pump inlet by the vacuum pump unit and in a second state gas can be evacuated from the pump outlet to the chamber to reduce pressure at the pump outlet.A method of controlling the vacuum pump system (10; 36; 46) comprises a first step of pumping gas from the chamber to the pump inlet using the vacuum pump unit and a second step of allowing gas to be evacuated from the pump outlet to the chamber to reduce pressure at the pump outlet.