A liquid ring gas pump having two axially adjacent, serially connected stages. Gas to be compressed is admitted to the first stage of the pump solely via a first stage inlet opening at the end of the first stage remote from the second stage. The sole outlet for gas to leave the first stage in order to flow to the second stage is a first stage outlet at the end of the first stage adjacent to the second stage. A residual gas outlet is provided at the end of the first stage remote from the second stage for allowing compressed gas that is unable to exit via the first stage outlet to exit via the residual gas outlet and re-enter the first stage via a residual gas inlet also provided at the end of the first stage remote from the second stage and located after the first stage inlet but before the first stage outlet in the direction of rotor rotation.