The kinetics and mechanism for the reaction of H with Si3H 8 have been investigated using various theoretical, methods including CCSD(T)/6-31.1++G(3df,2p)//B3LYP/6-311++G(3df,2p), G2M(RCC2), and CCSD(T)/ 6-31.1.++G(3df,2p)//CCSD/6-31 l.+G(d,p). The results obtained by the latter method show that H abstraction from a primary Si-H bond and a secondary Si-H bond leads to the formation OfIi-Si3H7 and J-Si 3H7 products, with 3.8 (TS1) and 3.2 (TS2) kcal/mol barriers, respectively. Significantly, the hydrogen substitution of SiH 3 and Si2Hs groups by attacking at the central Si atom via TS3 (3.3 kcal/mol) and a terminal Si atom of Si3H8 from side and end on (via TS4, 4.2 kcal/mol and TS5, 6.3 kcal/mol), were found to give SiH3 + Si2H6 and SiH4 + Si 2H5 products, respectively. The heats of formation of Si3H8, n-Si3H7, and i-Si 3H7 at 0 K are predicted to be 32.3 ± 1,2, 68.6, and 66.6 kcal/mol, respectively. These values are in good agreement with the experimental and other theoretical, values. The rate constants and branching ratios for the four product channels of the title reaction have been calculated by the transition state theory with Eckart tunneling corrections over a wide temperature region of 250-2500 K. These results may be employed for simulations of catalytic and plasma-enhanced chemical vapor deposition processes of a-Si:H films.