Dissociative adsorption of monomethylsilane (MMS), a promising precursor gas for low-temperature SiC, has been investigated on Si(100) by using temperature-programmed desorption (TPD) method after its comparison with H/ and C2H2/Si(100) surfaces. For both MMS/ and C2H2/Si(100), the beta(1) peak from SiH species showed a shift to higher temperatures in the presence of surface C atoms, while two new peaks appeared separately for the two gases: gamma peak (similar to 640 degrees C) for the C2H2/ and delta peak (similar to 870 degrees C) for the MMS/Si(100). The gamma peak is suggested to be from hydrogen desorption from SiH species at which a C atom is inserted to its backbond. The delta peak is related to desorption from surface CH, species. The absence of the gamma peak on MMS/Si(100) suggests absence of atomic exchange between surface C and substrate Si atoms on its adsorption.