We have investigated the spectroscopy and reaction of HCN (DCN) adsorbed on Si(100)-2×1 at Ts ≥ 100 K using HREELS, XPS, and UPS. HCN (DCN) formed dimers and/or polymers on the surface at 100 K and higher dosages (D > 4 langmuirs). The HREEL spectrum obtained after warming a 4.5-langmuir HCN dosed surface to 220 K resembles that obtained with a lower HCN dosage (D < 0.6 langmuir). Two major species, HCNH and CN, could be identified from this spectrum. The former species showed peaks at 160, 368, and ∼400 meV for HC=NH, CH, and NH stretching vibrations, respectively, while the latter showed a peak at 263 meV due to the C≡N stretching vibration. In the corresponding DCN experiment, the DC=ND stretching mode was observed at 124 meV. Annealing the sample at 560 K appeared to cause the reorientation of the CN radical from an end-on to a side-on adsorption geometry as evidenced by HREELS, UPS, and XPS analyses. At 600-800 K, the breaking of NH and CN bonds occurred on the surface. Above 1000 K, a mixture of silicon carbide and silicon nitride was formed after the complete dissociation of CH, NH, and CN bonds and the desorption of H species.