We describe here the initial bonding geometry and decomposition mechanism of HNO3 on Ge(100). A combination of high-resolution electron energy loss spectroscopy, Auger electron spectroscopy, and temperature programmed desorption experiments have been used to shed light on the details of this complex but technologically important system. It was found that HNO3 adsorbs molecularly at 190 K in a predominantly bidentate bonding configuration. When the substrate is warmed to 425 K, the molecule decomposes. In a process that involves two adjacent HNO3 adsorbates, H2O and NO desorb while O remains on the surface. Further heating above 600 K causes the O to desorb as GeO.