The interaction of H2 with the Si(111)-(7 × 7) surface is investigated by means of density functional slab calculations on a (4 × 2) reconstructed model surface. A viable mechanism for β1 desorption is identified, which involves thermally activated SiH2 units at adatom sites. This mechanism leads to adsorption and desorption barriers of 1.0 and 2.4 eV, respectively, in agreement with experiment. An explanation for the two components observed in the β1 peak of temperature-programmed desorption spectra is proposed. The lattice deformation energy at the transition state for desorption is large (not, vert, similar0.6 eV). If we assume that this remains in the surface after H2 desorption, the low translational energy of desorbing molecules which has been observed experimentally can in part be explained.