Centrifugal compressors find a huge number of applications in industry and in aero-engines; the detailed comprehension of the complex fluid-dynamic mechanisms occurring in these machines is crucial to improve their efficiency and their operating range. The paper presents a study on the impeller–vaned diffuser interaction in a high-performance compressor stage, in the frame of a wide experimental campaign devoted to the comprehension of unsteady flows in centrifugal compressor stages. The paper focuses on the best efficiency operating point. Data were collected in the impeller–vaned diffuser gap by applying a fast response probe. The impeller runs at 12500 RPM and the peripheral Mach number is 0.77. At first, data are reduced to highlight the main flow structures released by the impeller. CFD simulations — first verified against experimental data — were also performed to get detailed information of the flow field inside the impeller. The core of the paper is the discussion of the impeller–diffuser interaction. Results evidence the effect of the diffuser on the impeller in terms of static pressure and flow velocity. Moreover, the interaction process makes the average flow rate discharged by an impeller channel and the power exchange to be unsteady.

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