Abstract: The present invention relates to a double-multilayer monochromator. The monochromator comprises a base, a housing moveably mounted on the base and having in one side thereof at least one inlet window for the inlet of a X-ray beam and in another side thereof an outlet window for the outlet a monochromatized X-ray beam, support means mounted in the housing to support opposed mirror pairs, wherein each of the mirrors has a coated surface, each of said coated mirror surfaces being coated with a multilayer coating in alternating layers of a scatterer material (A) having a thickness dA and a spacer material (B) having a thickness dB. The present monochromator provides an enhanced purity of the synchrotron beam, where even and odd orders of higher harmonics are rejected. This is achieved in that the ratio dA:(dA+dB) on one of the facing mirrors of the mirror pairs is about 1:2 and the ratio dA:(dA+dB) on the other of the facing mirrors of the mirror pairs is about 1:3.