Several of the methods of explaining and evaluating the slip factor of a centrifugal impeller with straight radial blades are presented. The slip factors are calculated by these methods and compared to the experimentally observed characteristics of an impeller with 19 blades. It is shown that the slip factor depends on the gas-flow rate, the rotational speed, and the impeller-tip diameter. It is concluded that the slip-factor characteristics are better described by means of a constant leaving angle of the flow relative to the impeller and that the leaving angle is only a function of the impeller geometry.