We present a spectrometer design candidate for the X-ray Multi-Mirror (XMM) observatory, being planned by the European Space Agency (ESA) as a long-lived large-area array of telescopes. The science requirement of moderate resolution (E/ΔE ~ 100) spectroscopy in a two octave region (0.5-2 KeV) with extremely high throughput (effective area > 500 cm2) results in the use of grazing incidence reflection gratings. Due to the low image quality of the telescopes (~ 1 minute of arc), the grating dispersion must be maximized by use of the classical grating mount in which the spectrum is dispersed within the plane of incident radiation. Due to the small field of view afforded by the x-ray telescopes, the gratings must be situated in the converging beam at the exit of the telescope. A spectrometer module consists of a thin-foil conical mirror telescope, a stack of plane varied-space reflection gratings and an imaging proportional counter. This system is analyzed on the basis of dispersion, geometric aberrations and efficiency. At a spectral resolution of 0.15 Å, a twenty module XMM would attain an average effective area of ix, 900 cm2, reaching twice this value at the peak wavelength (15 Å). Similar throughput is obtained in second order centered at 7.5 Å, the two spectral orders separated by the non-dispersive energy resolution of the proportional counter. Continuous spectra are obtained in the 6-25 Å band (0.5-2 KeV), and can be extended to 45 Å if desired by tuning of the grating. The instrument sensitivity is sufficient to allow the first spectral detection of soft x-ray features in external galaxies, with access to an estimated population of several hundred active galactic nuclei. Such observations will expand vastly the roles feasible for spectroscopy in x-ray astrophysics, marking the beginning of a new era in space astronomy.© (1986) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.