This Ph.D. thesis deals with the experimental investigation of the unsteady three dimensional flow in a centrifugal compressor vaned diffuser by means of the LDV (Laser-Doppler-Velocimeter) measurement technique. By the analysis of extensive measurements carried out in the high subsonic flow regime of the experimental compressor a contribution to a better insight view and understanding of the complex flow in the vaned diffuser is accomplished. Additionally, the performed two and three dimensional LDV measurements complete and extend the existing experimental data base of centrifugal compressor flow.In order to perform LDV measurements under the difficult conditions in a narrow channel width diffuser (16.8 mm), it has been essential to adapt a commercially available LDV system. These adaptations implied improvements of the signal-to-noise ratio of the raw LDV bursts, Additionally a new sophisticated and transparent data acquisition and processing methodology was established. In a further development phase, the LDV measurement system, was extended in order to perform measurements and analysis in unstable operating points with Rotating Stall and Mild-Surge.Measurements m $tahle operating points have been performed at the best point and at part load with different circumferential Mach numbers. The impeller exit flow and the flow along the diffuser midline has been measured. Area measurements have been done in several planes parallel and normal to the impeller axis. The time averaged and time resolved flow has been analysed. Hence LDV data has been phase averaged and in the case of time averaged analysis the data was averaged afterwards arithmetically. The typical Jet/Wake structure of centrifugal compressor impeller flow is confirmed for this impeller. The time resolved viscous wake mixing is presented. The wake mixing is completed mainly behind the diffuser throat. Area measurements in the diffuser throat show already the presence of a secondary flow vortex.At. unstable operating points measurements have been performed with Rotating Stall and Mild-Surge. The raw data has been processed with a Multiple-Conditional Sampling data processing method, During Rotating Stall two different flow regions can be distinguished. A cell-flow region with slight badeflow from the diffuser into the impeller and a sound-flow region in the remaining impeller exit cross section. With the point-wise measuring LDV system it. was possible to capture the channel vortex in the rear part of the diffuser channel during Rotating Stall.In Mild-Surge the harmonic volume flow fluctuations are shown in the velocity profiles, which results in backfiow at the low circumferential Mach number 0.4.Part of this work are QuickTime animations of the diffuser flow, which give a comprehensive insight into the unsteady three dimensional flow in a varied centrifugal compressor. The animations axe on CD-ROM or may be downloaded from the web (www.lsm.ethz.eh).