The mechanisms and kinetics for the thermal decomposition of SiH2 + and SiH3 + ions, and related reverse reactions involving their ion fragments have been investigated for the first time by using ab initio molecular orbital and variational RRKM calculations. Geometries of all species involved the title reactions have been optimized by the CCSD(T), CCSD, and MP2 methods with the Aug-CC-pVTZ basis set while their single point energies have been refined by the CCSD(T)/CBS calculation for all the three levels for comparison. The barrierless processes involved have been calculated at the CASPT2//CASSCF/Aug-CC-pVTZ level of theory. The potential energy surfaces indicate that the SiH2 + and SiH3 + ions can eliminate an H atom via barrierless processes with dissociation energies of 48.4 and 91.5 kcal mol−1, respectively, or eliminate an H2 molecule via tight transition states at TS1 (38.5 kcal mol−1) and TS3 (59.9 kcal mol−1) giving H2 + Si+ (21.3 kcal mol−1) and H2 + SiH+ (36.7 kcal mol−1), respectively, at the CCSD(T)/CBS//CCSD(T)/Aug-CC-pVTZ level of theory. The rate constants for the reactions predicted for a wide range of T,P-conditions reveal that they increase with rising temperature and pressure and H2 elimination is the major channel in each of the decomposition reactions. The relative energies predicted by different methods agree closely; the predicted heats of formation for various species are in good agreement with available experimental values. The predicted rate constants for the forward and reverse reactions may be employed for kinetic modeling of PECVD processes.