We have investigated the adsorption and thermal decomposition of disilane on Si(100) (2 x 1) using infrared absorption spectroscopy. We demonstrate that at room temperature, disilane dissociatively adsorbs onto the surface with the dimer bond unbroken, to produce mono-, di-, and tri-hydride species. At low coverages, the disilane molecule adsorbs on the surface without breaking the Si-Si bond of the molecule. Thermal annealing following room-temperature adsorption of disilane produces a hydrogen-terminated adatom dimer (HSi-SiH) and an isolated monohydride species. We suggest that these hydride species are generated through the rupture of the substrate dimer bonds. Hydrogen desorption from the isolated monohydride occurs at much lower temperature than that from the adatom dimer.