The mechanism of the SiHs + SiHs reaction was studied by means of time resolved mass spectrometry. SiH3 radical was generated by the ArF laser photolysis of C2CU in SiH4/He mixtures, and the products were detected by a low-energy electron impact ionization mass spectrometer. The rate constant derived from the decay rate of SiH3 agreed well with that obtained from the rise rate of Si2Ho. The overall rate constant of the reaction at 297 ±2 K was determined to be (9.5 ±3.5) × 10 -11 cm3 molecule"1 s~'. H2 could also be detected as a product of the SiHa + SiHs reaction, and the yield of H2 was measured to be 11 ±4%. The other major product of the SiHs + SiHs reaction was SiH2, which rapidly reacts with SiRt to form Si2Ho. It was confirmed by the addition of a large amount of H2 as a SiH2-trapping reagent that Si2Ho detected at p = 5 Torr was exclusively produced via the SiH2 + SiKj reaction. On the basis of these observations, the branching ratios for the decomposition of vibrationally excited Si2H6 produced by the SiHa + SiHa reaction were estimated. These results were compared with conventional RRKM calculations.