The ROSAT position sensitive proportional counter is a multi wire X-ray detector which uses a mixture of Argon, Xenon and Methan as counter gas. In the X-ray energy band from 0.1 keV to 2.2 keV, this gas shows several absorption edges, caused by Argon-L and Xenon-N shells between 100 eV and 350 eV, and Xenon-M shells between 670 eV and 1150 eV. To measure the anode pulse height as a function of X-ray energy, we set up an X-ray spectrometer, using a transmission grating with 1000 lines per mm and a high intensity X-ray continuum source. The spectrometer had a resolution of 0.3 A. Besides the well known discontinuity of the pulse height at the Argon-L III/II edges, we could identify almost all Xenon edges in our energy range. Between the edges, the anode pulse height is proportional to the X-ray energy. At the edges, the pulse height drops, corresponding to an energy loss of a few electron volt. We will describe the measurements and present the results.