The Si(100) and (111) surfaces have generally been classified as relatively inert toward molecular hydrogen adsorption. In the present work, we show using surface second harmonic generation that the sticking probability increases dramatically for slightly misoriented substrates, which are of interest in many integrated circuit applications, At temperatures lower than 30-50 K below where desorption becomes significant, defect sites serve as adsorption conduits from which hydrogen atoms diffuse onto the rest of the surface. The adsorption rate therefore has the activation energy of surface diffusion, found to be 0.78 eV on Si(100) and 1.50 eV on Si(111). The small energy barrier and pre-exponential factor (similar to 10(-11) cm(2)/s) for diffusion on (100) seem to result from the need to break Si-Si dimer-pair bonds prior to hopping of an H atom. At higher temperatures, the adsorption mechanism changes, and H-2 sticks to both surfaces with a fairly large probability near 0.1.