In this article, we present calculated energies for the abstraction of hydrogen from silicon monohydride and silicon dihydride surface bonding units by atomic hydrogen obtained using ab initio configuration interaction theory. Three and four silicon atom clusters are used to model the dihydride and monohydride units, respectively. Heats of reaction and activation energy barriers are calculated, including the vibrational energies of the initial, final, and transition states. Hydrogen abstraction from a Si-H unit (H+Si4H10-->Si4H9+H-2) is found to be exothermic by 9.4 kcal/mol with a transition state energy barrier of 5.5 kcal/mol when H approaches along the surface normal. The dihydride abstraction reaction, H + Si3H8-->Si3H7 + H-2, is exothermic by 7.7 kcal/mol and has an energy barrier of 7.3 kcal/mol when H is approaching along Si-H axis. The barrier is larger for hydrogen atom approaching along the surface normal. The larger barrier for abstraction from a dihydride unit is consistent with our experimental observation of a preferential reduction in monohydride bond concentrations when hydrogenated silicon films are exposed to atomic hydrogen during plasma deposition.