We have studied the dereconstruction of Si(111)2×1 upon hydrogen adsorption by means of first-principles total energy and molecular-dynamics calculations. We find that the (1×1) bulk-terminated surface structure becomes energetically favored over the (2×1), starting from a coverage θ~0.3. This is consistent with LEED results showing that the dereconstruction is completed at θ~0.5. A path for this conversion has been determined via direct simulation at θ=0.5. This has a rather low energy barrier, ~0.10–0.15 eV/(surface atom), and should therefore occur with significant probability during hydrogenation of the surface.