Adsorption of D2 on Si(100) has been investigated using molecular beam techniques. The dependence of the D2 sticking coefficient, S, on surface temperature, Ts, and nozzle temperature, Tn, has been characterized. The sticking coefficient increases gradually in the range 300 ≤ Tn ≤ 1040 K. With an incident translational energy of ∼ 65 meV, S rises with increasing Ts from a value insignificantly different from the null level to a value of (1.5 ± 0.1) × 10−5 at Ts = 630 K. Having established that S increases with both increasing molecular energy and increasing Ts, we demonstrate directly that the adsorption of molecular hydrogen on Si is activated and that lattice excitations play an important role in the adsorption process. The implications of these results for the calculation of the SiH bond strength are discussed.