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 d_A and a spacer material (B) having a thickness d_B. 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 d_A:(d_A+d_B) on one of the facing mirrors of the mirror pairs is about 1:2 and the ratio d_A:(d_A+d_B) on the other of the facing mirrors of the mirror pairs is about 1:3.

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