Three-dimensional, viscous, and compressible flow calculations have been performed in order to improve the performance of a radial compressor diffuser with splitter. All calculations were carried out applying a commercially available finite-volume Navier-Stokes code. Diffuser design procedure, grid generation and boundary conditions are described in detail. For each diffuser type several operating points ranging from the surge limit to choke were considered. Velocity distributions showing regions of separation are discussed for different diffuser geometries. The downstream evolution of the pressure recovery coefficient is given along both diffuser channels. Calculations have been verified by experimental investigations on full scale diffuser prototypes providing pressure values at defined locations.