The mechanical and performance test evaluation of an experimental high pressure ratio single-stage centrifugal compressor incorporating a rotating vaneless-vaned diffuser is discussed. The test rig employed was a turbodrive configuration with the centrifugal impeller driven by a directly-coupled air turbine. The rotating diffuser was supported on a separate shaft operating at approximately 25% impeller speed and directly coupled to a high-speed water brake. Mechanical operation of the compressor rig was satisfactory. Best overall test performance achieved at design speed was a total-total pressure ratio of 7.7 with an adiabatic efficiency of 72.3% (average stage exit Mach number of 0.27). A broad flow range ratio of 1.25 between choke and surge was obtained with the diffuser rotating at design speed.