The photochemistry of Si2H6 adsorbed on H terminated Si(100) has been investigated using the time-of-flight (TOF) technique and high resolution electron energy loss spectroscopy (HREELS). Intact Si2H6 desorbs via a photochemical mechanism during illumination with either 5.0 or 6.4 eV photons. Two cross sections differing by two orders of magnitude are required to describe photodesorption of the first weakly chemisorbed monolayer. It is likely that molecules adsorbed in this state adopt one of two orientations. We suggest that, as in the Antoniewcz model, the initial excitation involves temporary electron trapping and that the molecular orientation dependence of this process is reflected by the two cross sections. Photofragmentation is initiated by 6.4 eV photons but not by 5.0 eV photons and results in the desorption of mostly SiH4 and H-2. TOF distributions corresponding to these two products are bimodal. The fast component corresponds to those fragments that are ejected with sufficient kinetic energy to overcome the desorption barrier directly whereas the slow component represents those fragments that become trapped and then thermally desorb. While the absence of fragmentation at a photon energy of 5.0 eV is consistent with the fact that the threshold for direct excitation is found to be at 5.2 eV, there is strong evidence that an indirect mechanism, again involving the formation of a temporary anion, is responsible for fragmentation. Specifically, HREELS shows that there are two resonances centered at 1.5 and 2.7 eV above the vacuum level. It appears that desorption can be initiated by electron trapping in either of these two resonances whereas fragmentation only occurs when a hot electron is trapped in the higher energy resonance. In common with photodesorption of intact Si2H6, photofragmentation is best described by two cross sections, although in this case they differ by a remarkable three orders of magnitude. There are at least two major Si2H6 fragmentation channels. It is proposed that one of these yields SiH4, Si and H-2 when a short-lived electronic excitation causes the nuclei to accelerate along a reaction coordinate leading to these products whereas the other yields SiH4 and SiH2 as a result of randomized vibrational excitation.