The kinetics of the thermal decomposition of silane gas during epitaxial growth on the (111) face of single crystal silicon were studied by modulated molecular beam mass spectrometry over the temperature range of 1000 K -1440 K and with beam intensities from 2 x 20 15 to 2 x 10 16 molecules/cm 2 -s.An abrupt change in the apparent reaction probability was observed at -1130 K, coinciding with a known surface structural transformation at about the same temperature. The molecular beam data for temperatures higher than 1130 K were used for construction of a reaction model. Additional experiments were conducted to fix certain features of the reaction mechanism. An attempt was made to measure the rate constant for desorption of unreacted silane molecules from the surface. The residence time of silane on the surface was below the sensitivity limit of the technique « 5 ~s), which means that SiH 4 desorption is not rate-limiting. Application of a simultaneous modulated beam of SiH4 and a steady beam of SiD4 did not produce HD molecules, thus ruling out hydrogen atom production in the surface reaction mechanism. According to the proposed model, silane molecules which do not reflect from the surface undergo a branched reaction with two types of sites on the surface, producing two bound SiH 2 molecules in each chemisorption event. Subsequent surface decomposition of the adsorbed SiH 2 molecules occur with different pre-exponential factors,' although with similar activation energies of about 17 kcal/mole for the two types of sites.