In assessing the quality of two instruments for particle size analysis, a diffraction pattern analyser and an X-ray sedimentation centrifuge have been used in multiple analysis of two size distributions of spherical and irregularly shaped particles. As expected the distributions differ due to shape influence caused by different particle characteristics and due to different evaluation algorithms in the case of spheres. The reproducibility of the diffraction data is approximately five to ten times higher than the corresponding centrifugal data. Different mixtures of the basic materials have been analysed and the size distributions thus obtained have been compared with the distributions calculated from the basic data. The experiments showed that the X-ray centrifuge determines the size distributions, the size dependent mixing ratio and the overall mixing ratio with a comparatively small error The measurement showed that the mass balances applied represent a sensitive test on instrument performance.