Gas-phase reactions of SiHx with Si2Hy (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6–311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n-Si3H7 and i-Si3H7. For example, SiH2 can associate with Si2H5 producing n-H2SiSiH2SiH3 exothermically by 55.8 kcal/mol; SiH3 can undergo addition to H2SiSiH2 to produce n-Si3H7 or associate with H3SiSiH barrierlessly forming i-Si3H7; whereas SiH can insert into one of the Si─H bonds of Si2H6 to give excited n-Si3H7. Similarly, H2SiSiH and SiSiH3 can undergo insertion reactions with SiH4 producing n/i-Si3H7 intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low-energy channel products have been calculated for the temperature range 300–1000 K by variational RRKM (Rice–Ramsperger–Kassel–Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma-enhanced chemical vapor deposition growth of a-Si:H thin films under practical conditions.