The thermal unimolecular decomposition of SiH4 + ion and its related reverse reactions, SiH3 + + H and SiH2 + + H2, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH4 + can mainly occur via a barrierless channel giving SiH2 + + H2 lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH3 +···H complex to be followed by a barrierless decomposition yielding SiH3 + + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH4 + → SiH2 + + H2 is predominate under PEVCD conditions. For H- and H2-capturing by SiH3 + and SiH2 + ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiHx + (x = 2–4) ions are in close agreement with available thermochemical data.