The reaction of disilane, Si2H6, with the Si(100) and Si(111) surfaces has been examined with supersonic molecular beam scattering techniques. The emphasis has been on elucidating the reaction mechanism operative under conditions leading to steady-state Si epitaxial growth. Two reaction mechanisms have been identified : (i) complete pyrolysis to form two adsorbed Si atoms and gas phase hydrogen; and (ii) a reaction forming one adsorbed Si atom, gas phase hydrogen, and silane, SiH4, as a gas phase product. The relative predominance of these two channels is sensitive to surface structure, adlayer composition, and incident kinetic energy. In particular, only complete pyrolysis is observed on the clean Si(100)-(2 X 1) and Si(11)-"(1 X 1 )" surfaces. The silane production channel, on the other hand, is observed on the Si(111)-(7 X 7) surface, and on the Si(100)-(2 X 1) surface in the presence of a finite coverage of either adsorbed hydrogen or phosphorus atoms. Examination of the reaction dynamics reveals that the probability of complete pyrolysis increases with increasing incident kinetic energy. Angular-resolved measurements of the scattered SiH4(g) product on the Si(111)-(7 X 7) surface suggest that silane is formed from the reaction of a chemisorbed intermediate. Comparison of the reaction probability of SiH4 and Si2H6 on the Si(111)-(7 X 7) surface as a function of incident kinetic energy suggests a similar decomposition mechanism for these two molecules, namely, Si-H bond activation. In this scenario, SiH4(g) is formed via unimolecular thermal decomposition of an adsorbed Si,HS(a) species.