The interaction between NH2CHO (ND2CHO) and Si(100)-2×1 has been investigated using HREELS, UPS, XPS, and TDS. At 100 K, formamide molecules randomly adsorb on the surface but become ordered, likely with the O-atom attached to the surface, as the 2 L NH2CHO dosed Si(100) surface is warmed to 200 K. At this temperature, the formation of some Si-H species is indicated by the appearance of the 256 meV peak due to the Si-H stretching vibration in HREELS. When the sample is annealed at 450 K, more H species are formed, accompanied by the appearance of OH. In addition, a peak at 172(160) meV appears in HREELS, which can be attributed to the HN═C (DN═C) and/or HNCO (DNCO) species. The latter is more clearly evident in the UPS data. At 550 K, the breaking of CH and NH bonds continues and the OH and H dominate the adspecies. Further annealing the sample at higher temperatures causes the desorption of NH3 (ND3) and HCN (DCN) species and the formation of the Si-O-Si complex as indicated by the 129 meV peak in HREELS due to the asymmetric stretching mode of the Si-O-Si species. Above 800 K, the Si-H stretching mode at 260 meV decreases dramatically because of the desorption of hydrogen, as confirmed by the result of TDS.