For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat-CVD) processes using small silanes as precursors, disilanyl radical (Si2H5) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a-Si:H film growth on large-area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si2H5 producing smaller silicon hydrides including SiH, SiH2, SiH3, and Si2H4, and the related reverse reactions involving these species by using ab initio molecular-orbital calculations. The results show that the lowest energy path is the production of SiH + SiH4 that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si2H5. Additionally, the dissociation energies for breaking the SiSi and HSiH2 bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH2, HSiSiH2, and Si2H4(C2h) at 0 K by using the isodesmic reactions, such as 2HSiSiH2 + 1C2H6→1Si2H6 + 2HCCH2 and 1Si2H4(C2h) + 1C2H6 → 1Si2H6 + 1C2H4. The results of SiH (87.2 kcal/mol), SiH2 (64.9 kcal/mol), HSiSiH2 (98.0 kcal/mol), and Si2H4 (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si2H5 and the related bimolecular reverse reactions have been predicted and tabulated for different T, P-conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH4 product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH2 + SiH3, HSiSiH2 + H2, and Si2H4(C2h) + H under T, P-conditions typically used in PECVD and Cat-CVD. © 2013 Wiley Periodicals, Inc.