The stability of trimethyl indium on the surface of a Si(100)(2 × 1) single crystal has been investigated with UPS, XPS and HREELS. The Si dangling bond on Si(100)(2 × 1) was found to play an important role in the adsorption of TMIn, probably through the interaction with In 5pz orbitals and thus causing a distortion and/or the dissociation of the In-C bonds. However, the In-C bond breaking occurred to a much smaller extent on the (100) surface than on Si(110) and Si(111)(7 × 7). In the case of the (111) surface, TMIn attacks the adatoms before the rest-atoms. When 1 L dosed samples were annealed at 520 K, the In-C stretching vibration mode at 62 meV could still be observed in HREELS for TMIn on Si(100)(2 × 1), but not on the other two surfaces. Above 520 K, the In-C bond cracking was complete. Meanwhile CH bond breaking also began to occur on the surface, which continued as the sample was further annealed at higher temperatures. At 950 K, only C species left on the surface following the decomposition of TMIn and the desorption of the In and H species.