Mol. hydrogen exhibits an exceedingly low dissociative sticking coeff. on the (100) and (111) faces of silicon. This finding would suggest that the mol. experiences a large barrier (>100 kJ/mol) when it attempts to adsorb. However the assumption of such a large barrier is in conflict with the known desorption activation energy and the est. bond strength of hydrogen atoms to silicon surface atoms. In recent expts. it has been obsd. that the desorbing mols. do not carry away a large extend of excess energy as translational energy nor in their internal degreess of freedom. This apparent contradiction can be resolved if one considers that the dangling bonds on the silicon surface are localized and that hydrogen adsorption induces significant alterations of the surface structure. Hence, hydrogen adsorption is expected to be activated in degrees of freedom localized in the surface. This proposition has been substantiated by the observation of an increasing adsorption probability with increasing sample temp. Recent total energy calcns. lend theor. confirmation to this model.