The translational energy of D₂ desorbed from Si (100) and Si (111) surfaces was measured and found roughly equal to the thermal expectation at the surface temp. (Ts). By combining these results with previously measured internal state distributions, the total energy of the desorbed mols. is approx. equal to the equil. expectation at Ts. Thus adsorption expts. which suggest a large energetic barrier are at variance with desorption expts., which exhibit a trivial adsorption barrier, and the applicability of detailed balance for this system needs to be reexamd.