For liquid surface studies a monochromatic beam of relative bandwidth between 0.1% and 1% at a variable angle in the vertical plane between 0-degrees and 10-degrees is needed. The beam should be like a sheet some tens of mu-m thick and some mm wide, and as intense as possible. We discuss a monochromator made of a beryllium mosaic crystal using the (002) reflection in Laue geometry placed in undulator beams of DORIS III at the Hamburger Synchrotronstrahlungslabor and of the European Synchrotron Radiation Facility. An analysis of the diffraction properties in terms of mosaic spread, heat load, optical quality of the monochromatic beam, and, in particular, the expected flux, gives an optimum value of about 0.2 mrad for the mosaic spread. We show by experiments that presently available crystals a few hundred mu-m thick have the required quality and are capable of reflecting up to 45% of the useful x rays, in full agreement with diffraction theory including secondary extinction. Heat load experiments reported elsewhere in this conference prove that Be crystals can withstand high beam power density and that the thermal deformation is small compared to the mosaic spread. The results of the flux calculations are most encouraging: a gain of order 10(3) to 10(4) can be expected as compared to existing liquid surface spectrometers.