The SPECTRUM - X-GAMMA mission is being developed by the Space Research Institute (IKI), USSR, together with many other countries and is scheduled for launch in 1993 ( Sunyaev,1990; Schnopper,1990). Mission objectives include broad and narrow band imaging spectroscopy over a wide range of energies from the EUV through gamma rays, with an emphasis on studying galactic and extragalactic X-ray sources. The Danish Space Research Institute (DSRI) and IKI will provide two thin-foil X-ray telescopes (SODART), each with an aperture of 60 cm and focal length of 8 m. They are designed to have a half-power width of less than 2 arc minutes and will have collecting areas of 1700, 1200 and 100 cm2 at 1, 8 and 20 keV, respectively. Images and polarization will be recorded by position-sensitive proportional counters. Moderate resolution spectroscopy will be done by the segmented solid state detector SIXA (Silicon X-ray Array), designed and to be constructed by a consortium in Finland, Denmark and USSR. Finland will have the main responsibility in financing and delivering the detector. The Institute of Electromechanics in Moscow will provide its passive cooling system (110 K). The detector will consist of 19 segments (Si(Li)), each with a diameter of about 8 mm. The spectral resolution of 160 eV (at 6 keV), combined with the large collecting area, provide good opportunities for time-resolved iron line spectroscopy ( 6-8 keV). The potential observing program includes stellar coronae, cataclysmic variables and X-ray binaries, accretion discs and coronae of neutron stars and black hole candidates, supernova-remnants, active galactic nuclei, clusters of galaxies and the diffuse cosmic X-ray background. We demonstrate the instrument’s power through some astrophysical simulations.