The adsorption of ethanol-d(0), -d(3), and -d(6) on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range ( < 1450 cm(-1)) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C-3 upsilon axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the observed normal mode frequencies;