The room-temperature (RT) adsorption and surface reactions of styrene on Si(I 00)2 x I have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2 x I at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2 x I reconstructed surface. The chemisorption of styrene on the 2 x I surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2 x I surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2 x 1, sputtered and oxidized Si(I 00) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(l 00)2 x I upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.