The surface chemistry of trimethylgallium (TMGa), dosed on Si(100) at 115 K, has been studied using SSIMS, TPD, and AES. The results are compared with CD3I to clarify the role played by methyl migration from Ga to Si and to identify subsequent decomposition channels. For TMGa, the TPD products are multilayer parent (150 K), hydrogen (800 K), and atomic gallium (925 K). SIMS of TMGa suggests Ga-C bond cleavage and attachment of the methyl group to Si starting as low as 115 K and going to completion at 420 K. When TMGa is coadsorbed with CD3I, observation of SiCH3+ and GaCH3CD3+ in SSIMS confirms the facile cleavage of Ga-C bonds and the formation of Si-C bonds. In agreement with previous studies, we find that carbon contamination of the silicon substrate is intrinsic to TMGa decomposition.