The mechanism of the SiH3 + SiH3 reaction was studied by means of time-resolved mass spectrometry. SiH3 radical was generated by the ArF laser photolysis of C2Cl4 in SiH4/He mixtures, and the products were detected by a low-energy electron impact ionization mass spectrometer, The rare constant derived from the decay rate of SiH3 agreed well with that obtained from the rise rate of Si2H6. The overall rate constant of the reaction at 297 +/- 2 K was determined to be (9.5 +/- 3.5) x 10(-11) cm(3) molecule(-1) s(-1). H-2 could also be detected as a product of the SiH3 + SiH3 reaction, and the yield of H-2 was measured to be 11 +/- 4%. The other major product of the SiH3 + SiH3 reaction was SiH2, which rapidly reacts with SiH4 to form Si2H6. It was confirmed by the addition of a large amount of H-2 as a SiH2-trapping reagent that Si2H6 detected at p = 5 Torr was exclusively prouduced via SiH2 + SiH4 reaction. On the basis of these observations, the branching ratios for the decomposition of vibrationally excited Si2H6 produced by the SiH3 + SiH3 reaction were estimated. These results were compared with conventional RRKM calculations.