The adsorption of disilane (Si(sub 2)H(sub 6)) on D terminated Si(100) has been investigated using photodesorption to probe the coverage over a range of surface temperatures. Incident Si(sub 2)H(sub 6) molecules can become molecularly (nondissociatively) chemisorbed after being trapped in a physisorbed precursor state. At temperatures higher than approximately 116 K, nearly all the physisorbed molecules desorb rather than chemisorb. Photofragmentation products are readily observed when chemisorbed Si(sub 2)H(sub 6) is irradiated with 6.4 eV photons but surprisingly, none are observed in the case of physisorbed Si(sub 2)H(sub 6). Consequently, Si can be deposited by reactions of the photofragmentation products with the surface, but only at temperatures lower than 116 K. The photodesorption cross section is also extremely dependent on the chemical nature of the Si(100) surface. It is high when the surface is terminated with a monodeuteride phase but much smaller when dideuteride species are present, suggesting that the presence of surface gap states is of importance. We consider how desorption might be initiated by electron attachment or by electron-hole recombination at the surface.