A double-crystal monochromator has been brought into operation at JET to measure absolute wavelengths and intensities of impurity spectral lines in the soft X-ray region from about 0.1nm to 2.3nm. It is capable of operating also during the deuterium tritium ( aetive) phase of JET because its deteetor is very effectively shielded against neutrons and hard X-rays. The spectrometer has been swept over a wide wavelength range, around characteristic line groups as well as monochromatic line scans.