The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field, focusing on the interaction between the impeller and the passage diffuser of a close-coupled transonic centrifugal compressor used in an aero engine. The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have an understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged, and spectral pressure distributions within the stage to validate the CFD. The work presented here shows the unsteady phenomena caused by the interaction and the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the blade passing frequency (BPF) of the diffuser, whereas in the diffuser both the BPF and the passage passing frequency (PPF), are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in the total pressure as a result of tangentially expanding pressure waves. Within the passage a decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.