The adsorption of D-2 On Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D-2 sticking coefficient S on surface temperature T-s and nozzle temperature T-n. The sticking coefficient increases gradually in the range 300 less than or equal to T-n less than or equal to 1040 K. The influence of increased upsilon=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on T-s is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5+/-0.1)X10(-5) at T-s=630 K. The decrease in the effective sticking coefficient beyond this T-s is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.