In this paper the transonic flow field in a vaned diffuser of a centrifugal compressor has been studied. Different geometries of the diffuser blades have been considered. Particularly with respect to the pressure field it is important to understand their influence on the development of the shock waves. For this problem a centrifugal compressor with high pressure ratio per stage has been examined. It is assumed that the impeller speed is so large that the flow may become transonic at the impeller exit. To calculate flows with shocks a time-marching method is used for solving the two-dimensional Euler equations in a conservative form. A MacCormack's second order difference scheme is applied. There are two important points which have been studied in this paper. Firstly the geometry of the blades as far as the thickness distribution is concerned is kept unchanged and only the stagger angle has been altered. The performance maps of these diffusers and their effect on the whole machine will be discussed. Secondly the results which have been obtained by using vanes with different thickness distributions will be presented. Mean values of the quantities used for the performance map will be shown. Wall pressure distributions as well as fields of isobars and of constant Mach numbers will be demonstrated to provide a better understanding of the pressure recovery in centrifugal compressor diffusers. © Martinus Nijhoff Publishers, Dordrecht and Freund Publishing House