The collision-induced associative desorption ͑CID͒ and abstraction ͑ABS͒ of D adatoms by H have been studied on the Si͑100͒ surfaces. D 2 CID exhibits a feature common to that of a thermal desorption from a dideuteride phase. HD ABS proceeds along an apparently second-order kinetics rather than a first-order kinetics with respect to surface D coverages. The ABS cross section is about 6 Å 2 , extremely large compared to the theoretical values. Both of the direct Eley-Rideal mechanism and the hot-atom mechanism are ruled out. A hot-complex-mediated reaction model is proposed for ABS and CID.