The surface chemistry of indium. tin oxide (ITO) has been investigated with Auger electron spectroscopy (AES) and high resolution electron energy loss spectroscopy (HREELS). A vibrating Kelvin probe (KP) with a graphite reference was used to monitor the absolute work function (fl of ITO as a function of chemical modification. The ITO was exposed in situ to molecular hydrogen (H-2), hot-filament-activated oxygen (O-2(*)), and hot-filament-activated deuterium (D-2(*)). The initial Phi of ITO was determined to be 5.2 eV, and surface chemical changes had strong effects on this value, as seen by KR Exposure of clean ITO to O-2(*) increased Phi to similar to5.6 eV, but the increase was short-lived. The changes in Phi over time were correlated with the uptake of carbon impurities in ultra high vacuum (UHV), as monitored by AES. The HREELS of ITO revealed significant hydrocarbon impurities. Chemical reduction of ITO produced a metallic surface and dehydrogenated the adsorbed hydrocarbons. Both re-oxidation of metallic ITO and oxidation of clean ITO temporarily removed adventitious carbon from the surface, but oxidized ITO adsorbed an even larger quantity of carbon over time.