Abstract: A supersonic centrifugal compressor comprises a bladed rotor which delivers fluid at an absolute velocity exceeding Mach 1.2 under rated conditions to an annular radial flow diffuser. The diffuser has a casing including spaced sidewalls and vanes defining intervane channels each having a throat section located downstream of the leading edges of the channel defining vanes. A pair of common secondary spaces are formed each in one of said sidewalls and communicate with parietal slots symmetrically formed in the sidewalls and opening into the channels. The slots are located and dimensioned to overlap the throat section in the flow direction and each has a cross-sectional flow area at least equal to half the flow area of the channel at the throat.