The effect of the diffuser vane setting angle on the performance of a centrifugal compressor was experimentally investigated. Seven different vaned diffusers were tested with the same impeller. In order to evaluate the vaned diffusers under the same flow range and diffuser inlet conditions, all the diffusers were designed to have the same throat area, the same diffuser height. and the same vane inlet position. The first tests were performed by a compressor with a volute. In this case, the diffuser outlet conditions were varied along the circumferential directions due to the nonaxisymmetric geometry of the volute. In the second tests, four of the seven vaned diffusers were tested using a compressor with a collector. In this case, the diffuser outlet conditions were more uniform along the circumferential directions compared to the tests using the compressor with the volute. The effect of the vane setting angle on the stage characteristics and diffuser performance as well as the effect of the circumferential distortion caused by the volute on the stage characteristics are presented. The impact of the incidence loss on the vaned diffuser performance is discussed using 1D vaned diffuser performance modeling. Comparisons of the 1D predictions and the tests results show the incidence loss has a strong influence on vaned diffuser performance.