Vacuum pump for pumping gas from a process chamber, the pump comprising: a rotor supported for rotation in a pumping chamber by a drive shaft, the shaft extending through a shaft bore in a wall of the pumping chamber extending transversely to the drive shaft, and a seal arrangement provided between the shaft and the transverse wall for resisting the passage of gas through the shaft bore, wherein on rotation of the rotor gas is pumped from a low pressure region at an inlet of the pumping chamber to a high pressure region at an outlet of the pumping chamber, and wherein the rotor and the transverse wall are spaced apart by an axial clearance along which back leakage of gas can flow from the high pressure region to the low pressure region, and an additional leakage path for back leakage is formed spaced away from the seal arrangement along which gas can flow without contact with the seal arrangement so that the amount of gas which contacts the seal arrangement during use of the pump is reduced.