Collision-induced desorption (CID) and Eley-Rideal (ER) type abstraction have been investigated in the reaction system, H(g) + D-ad/Si(100)--> D-2,HD, at surface temperature of 573 K where dihydride phase is unstable. Rate equations for CID were obtained based on a second-order kinetics with respect to doubly occupied Si dimers DSi-SiD, DSi-SiH, and HSi-SiH, which are considered as parent species of transiently formed dihydrides, DSiD, DSiH, and HSiH, respectively. The coverages of the doubly occupied dimers were determined as a function of H exposure time by means of temperature-programmed-desorption (TPD), and then the rate equations were calculated. As a result, the experimental D-2 and HD rate curves were reasonably fit with the rate equations formulated. This fact indicates that CID is induced when two dihydride species encounter during their propagation via dihydride-monohydride isomerization reactions. It was found that HSi-SiH does not play a role in CID of both D-2 and HD. Possible origins of this isotope effect were discussed in terms of quantum effects on associative desorption and diffusion processes. The HD rate curve due to direct ER abstraction channel was fit with a first-order kinetics in D-ad coverage for a low coverage regime where dangling bonds exist. However, for a high coverage regime where the surface is saturated, the HD rate curve was fit with DSi-SiD coverages, suggesting that D-ad that are paired up with H-ad in the same Si dimers are not abstracted.