We have investigated in-situ the adsorption and thermal decomposition of methylsilanes, SiHx(CH3)(4-x) (x = 1-3), on Si(100)(2 x 1), using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry. IRAS spectra revealed that at initial stages of adsorption, monohydride (-SiH) and CH3-substituted hydride species (-SiHx(CH3)(3-x)) are generated with monohydride species being dominant. We suggest that upon room temperature adsorption of methylsilanes, breaking of the Si-H bonds of methylsilane is favored over that of the Si-C bonds. It is found that the dissociative adsorption of SiH3(CH3) exhibits the second-order kinetics. Due to thermal annealing, surface species -SiHx(CH3)(3-x) are thermally decomposed to generate surface Si-H and Si-C bonds, and subsequently H-2 desorption from the Si-H bonds occurs.