Telescopic measurements of Mars between 4.40 and 5.13 μm at a spectral resolution (λ/Δλ) of 300 were made on August 19, 1988, UT at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. This wavelength region contains radiation from both solar reflection and thermal emission. Additionally, the Martian atmosphere has numerous atmospheric gas absorption features, dominated by the 4.2-μm CO2 fundamental band. The mars spectrum rise out of the 4.2 to 4.4-μm CO2 band on the long-wavelength side cannot be matched solely by atmospheric gas constituents. An absorption must be added at roughly 4.5 μm in order to decrease the reflectance rise and produce the 4.5-μm inflection which is present in the data. The location of this feature at the position of the 2&ngr;3 overtone of the SO2-4 anion indicates that the surface absorption is probably caused by sulfates on the Martian surface and/or in atmospheric dust. This is consistent with the known presence of sulfates on the Martian surface from the Viking results. An exact spectral match to a terrestrial sulfur mineral has not been made, but we suggest that the mineral on Mars has very weak band structure, probably due to an ion environment in the mineral with a high degree of electric field symmetry. Significant variation exists at 4.5 μm among the observed spectra for different locations on Mars. In order of strength, from strongest absorption to weakest, are Eastern Solis Planum, Argyre Basin, Eastern Tharsis, and Valles Marineris for the four regions measured at similar Mars atmospheric conditions. ¿ American Geophysical Union 19