A self priming gear pump 14 for pumping liquids containing gases is provided with a vent pipe 26 for releasing entrapped gases that would otherwise collect at the outlet side of the pump 14. The vent pipe 26 terminates within the pump housing 21 at a zone which is out of the path of the oil centrifuged by the gears 25 and in the region lying near the point of intersection of the gears 25. An additional inlet pipe 27 which directs liquid across the main inlet into the gap between the periphery of one of the gears 25 and the housing 21 may be provided to improve liquid sealing.