H, H2, and SiHx + (x ≤ 4) ions coexist under plasma-enhanced chemical vapor deposition (PECVD) conditions. We have studied the kinetics of their interactions by high-level quantum chemical and statistical theory calculations, and compared the results with classical Langevin values (∼2 × 10-9 cm3 molecule-1 s-1 independent of temperature). The results indicate that, for H capturing by SiHx + (x ≤ 4), both theories agree within a factor of 2-4, whereas for H2 capturing by SiHx + (x ≤ 3), the modern theory gives higher and weakly temperature-dependent values by up to more than one order of magnitude, attributable to reaction path degeneracies and increased entropies of activation. The heats of formation and structural parameters of SiHx + ions (x ≤ 5) in this work agree well with available experimental data. For practical applications, we have provided tables of rate constants for modeling various processes of relevance to the PECVD of a-Si:H films.