Abstract: A compressor comprising a housing having an axial intake and an annular outlet volute, an impeller mounted for rotation about a shaft axis, the impeller having a plurality of blades, an annular diffuser passage surrounding the impeller extending from a diffuser inlet to a diffuser outlet, the impeller hub being mounted in an annular impeller recess in a first wall member of the diffuser, wherein an annular diffuser recess is provided in the first wall member extending radially outwardly from a first end to a second end, the first end being located at a radially outer end of the impeller recess, the second end being located axially inboard of the first end of the diffuser recess and wherein the depth of the diffuser recess at the first end of the diffuser recess is at least 30% of the maximum depth of the impeller recess.