A centrifugal pump (10) includes a pump casing (12) within which an impeller (14) is mounted for rotation, cantilever fashion, on a shaft (16). The casing (12) has an axial inlet (20), and a peripheral volute (22) around the impeller (14) leading to an outlet (24). The impeller (14) has axially spaced annular sides (26, 28) with radially outwardly arranged, rearwardly curved, vanes (30) between the sides. Outwardly of the sides, clearing corresponding sides of the casing (12), auxiliary vanes (32, 34) are provided to generate a pressure gradient to prevent or counteract leakage of working fluid. Leading faces of the auxiliary vanes slope relative to a perpendicular to the sides, i.e. the leading edges are at an obtuse angle to the sides (26, 28).