Mid-infrared spectra (7.0 to 13.5 μ) of six different areas on the lunar surface have been obtained from an altitude of 31 km by using a 24-inch balloon-borne telescope. All spectra show significant departures from blackbody or graybody emission, in particular displaying a peak in emissivity that varies in wavelength from 8.11 to 8.37 μ. The position of this peak varies regularly with type of surface feature in the field of view. Laboratory studies show that the wavelength of peak emissivity in surface emission can be interpreted in terms of rock composition, and that the differences in peak emissivity from place to place result from differences in rock type. Barring some unanticipated systematic error, the data obtained so far indicate that the two circular maria, Imbrium and Serenitatis, are ultrabasic in composition, whereas the lunar crust appears to be basic in composition. This compositional heterogeneity may explain location of major mascons in the circular maria.