The adsorption and decomposition of disilane on Si(100)(2 x 1) was investigated using in-situ infrared (IR) absorption spectroscopy. The IR data demonstrate that upon room-temperature adsorption, disilane dissociatively adsorbs on the unsaturated dangling bonds of dimers with the dimer bonds unbroken, to produce mono-, di-, and tri-hydride species. At low coverages, dissociative adsorption without breaking of the Si-Si bond of Si2H6 is favored. Thermal annealing following room-temperature disilane adsorption produces the doubly-occupied adatom dimers (DOD, HSi-SiH) and isolated monohydride species. These hydride species are generated via the rupture of dimer bonds of the substrate. Hydrogen desorption from the isolated monohydride site occurs at lower temperatures than from the DOD site.