•The MESSENGER XRS measured ~1–10 keV X-rays from the surface of Mercury.•Surface abundances of Mg, Al, Si, S, Ca, Ti, and Fe were obtained.•Calibration measurements are presented for the reduction and analysis of these data. The X-Ray Spectrometer (XRS) that flew on the MESSENGER spacecraft measured X-rays from the surface of Mercury in the energy range ~1–10keV. Detection of characteristic Kα-line emissions from Mg, Al, Si, S, Ca, Ti, and Fe yielded the surface abundances of these geologically important elements. Spatial resolution as fine as ~40km (across track) was possible at periapsis for those elements for which counting statistics were not a limiting factor. Four years of orbital observations have made it possible to generate from XRS spectra detailed elemental composition maps that cover a majority of Mercury׳s surface. Converting measurements to compositions requires a thorough understanding of the XRS instrument capabilities. The ground and flight calibration measurements presented here are necessary for the reduction and analysis of the X-ray data from the MESSENGER mission.