The invention relates to an arrangement of a measuring window in a continuously operated X-ray analyzer (1), said analyzer being is used particularly for analyzing elemental contents in solid, liquid or slurry-like materials; which measuring window (2) separates the sampling space (3) containing the sample material to be measured and the measurement space (4) containing the measuring probe (11), and is sealed by a lid structure (6) arranged in the sampling space, said lid structure defining the measurement aperture (7) of the sampling space, in which case the lid structure defining the measurement aperture of the sampling space is provided with a sealing surface (8) of the measuring window, so that said surface is at least partly planar and at least partly curved.