A vacuum pump (10; 50; 60; 70) comprising a scroll pumping mechanism (11; 78) which comprises: two scrolls (20, 22) which are co-operable for pumping gas along a pumping channel from a scroll inlet (24; 25) to a scroll outlet (26; 27) of the mechanism on relative orbiting motion of the scrolls, a gas conduit (38; 62; 72; 38; 52) having an inlet (40; 54; 66; 74; 90; 94) at a first location of the pumping channel and an outlet (42; 56; 64; 72; 92; 96) at a second location of the pumping channel for allowing over-compression of gas above atmosphere at the first location of the pumping channel to be exhausted to the second location of the pumping channel, a one-way valve arrangement located in the gas conduit for allowing the passage of gas through the conduit from the conduit inlet to the conduit outlet only when a predetermined pressure differential between the first and second locations of the pumping channel is generated during roughing when the scroll inlet is at or close to atmosphere, characterised in that the scroll pumping mechanism comprises a first section adjacent the scroll inlet and a second section adjacent the scroll outlet and the pumping capacity of the first section is larger than the pumping capacity of the second section, and wherein the first location of the pumping channel is downstream of a transition between the first section and the second section, and wherein the one-way valve arrangement comprises two one-way valves located in the gas conduit (38; 62; 72; 38; 52) for resisting the passage of gas through the conduit from the conduit inlet to the conduit outlet when closed and allowing the passage of gas through the conduit when open. two scrolls (20, 22) which are co-operable for pumping gas along a pumping channel from a scroll inlet (24; 25) to a scroll outlet (26; 27) of the mechanism on relative orbiting motion of the scrolls, a gas conduit (38; 62; 72; 38; 52) having an inlet (40; 54; 66; 74; 90; 94) at a first location of the pumping channel and an outlet (42; 56; 64; 72; 92; 96) at a second location of the pumping channel for allowing over-compression of gas above atmosphere at the first location of the pumping channel to be exhausted to the second location of the pumping channel, a one-way valve arrangement located in the gas conduit for allowing the passage of gas through the conduit from the conduit inlet to the conduit outlet only when a predetermined pressure differential between the first and second locations of the pumping channel is generated during roughing when the scroll inlet is at or close to atmosphere, characterised in that the scroll pumping mechanism comprises a first section adjacent the scroll inlet and a second section adjacent the scroll outlet and the pumping capacity of the first section is larger than the pumping capacity of the second section, and wherein the first location of the pumping channel is downstream of a transition between the first section and the second section, and wherein the one-way valve arrangement comprises two one-way valves located in the gas conduit (38; 62; 72; 38; 52) for resisting the passage of gas through the conduit from the conduit inlet to the conduit outlet when closed and allowing the passage of gas through the conduit when open.