Gas-phase kinetics and mechanisms of SiH3 reactions with SiH4, Si2H6, Si3H8, and Si4H10, processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH3 reactions with these silanes occur by H abstraction, leading to the formation of SiH4 + SixH2x+1 (silanyl) radicals. For both Si3H 8 and n-Si4H10 reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH 4 + s-Si3H7 and SiH4 + s-Si 4H9, respectively. In the i-Si4H10 reaction, tertiary Si-H abstraction has the lowest barrier producing SiH 4 + t-Si4H9. In addition, direct SiH 3-for-X substitution reactions forming Si2H6 + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH3 reactions with the analogous CH3 reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si 4H10 isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.