Density functional theory calculations have been used to explore the mechanism of dissociative adsorption of silane (SiH4) on the Si(100)-(2 X 1) surface. Two reaction paths are described that produce silyl (SiH3) and hydrogen atom fragments adsorbed on the dimer dangling bonds. The energy barrier on the lowest energy path is 12-14 kcal/mol (depending on the details of the theoretical method used), while the barrier on the other path is about 17 kcal/mol. The initial step in both mechanisms is abstraction of a hydrogen atom from silane by an electron-deficient surface atom. It is also possible for the surface to react by forming a bond between the more electron-rich surface atom and the silane Si atom. This latter reaction path has a prohibitively high barrier (39 kcal/mol), and it leads to different products (adsorbed SiH2 and elimination of H-2) . These results are discussed in the context of Si film growth kinetics, ultrahigh vacuum studies of silane adsorption and other theoretical studies of silicon surface chemistry.