The chemisorption of bis(trimethylsilyl)methane (BTM, CH2[Si(CH3)3]2) and dodecamethylcyclohexasilane (DCS, Si6(CH3)12) on clean Si(100) surfaces has been studied by C 1s core-level and valence-band photoemission spectroscopy. Our model for the deposition of carbon by BTM involves decomposition into a –CH2Si(CH3)3 surface moiety for room-temperature adsorption, which further decomposes upon annealing to 550 °C to form a surface terminated primarily by CHx units. DCS deposits almost three times as much C on the surface as BTM. The data are consistent with DCS undergoing a ring opening and bonding to the surface as polydimethylsilane chains. Annealing both adsorbates to 950 °C causes a large decrease in the C 1s signal due to the fact that Si segregates to the surface at temperatures above 900 °C. The valence-band photoemission of Si(100) dosed with DCS at 950 °C is in good agreement with that of β-SiC, whereas the analogous BTM spectrum deviates significantly.