Focusing characteristics of diamond crystal x-ray monochromators. An experimental and theoretical comparison. M. Sánchez del Río * , G. Grübel * , J. Als-Nielsen *+ and M. Nielsen + *) European Synchrotron Radiation Facility, BP220 38043 Grenoble Cedex 9 (France) +) Risø National Laboratory, Roskilde (Denmark) Abstract. Perfect crystals in transmission (Laue) geometry can be used effectively for x-ray monochromators and moreover perfect Laue crystals show an inter-esting focusing effect when the incident beam is white and divergent. This fo-cusing is directly dependent on the beam divergence and on the diffraction profile width of the crystal. The aim of this work is to study whether this prop-erty can be used for focusing a synchrotron x-ray beam, and to obtain quanti-tatively the dimensions of the resulting monochromatic beam. We have experimentally measured the size of an undulator beam after diffraction from a diamond crystal in Laue geometry, and we analyze and explain the results by comparison with ray-tracing simulations.