Flash pyrolysis of silane, SiH4, and disilane, Si2H6, diluted in He or Ar (1%), was carried out at temperatures ranging from similar to 700 to similar to 1500 K. After a short reaction time of similar to 20 mus, the initial products were isolated in a supersonic molecular beam and detected by single vacuum ultra-violet (VUV) photon (lambda = 118 or 121 nm) ionization time-of-flight mass spectrometry (TOFMS). Initial decomposition and reaction products, both free radical intermediates and stable species, were directly observed, which included SiH2 and Si2H4.