Kinetics of boron deposition by hydrogen reduction of BCl3 was investigated in a dual impinging-jet CVD reactor. Measurement of the effluent stream composition by an FT-IR allowed detailed analysis of the reaction kinetics. Formation of the intermediate BHCl2 was found to take place both in the gas phase and also through a surface reaction involving three sites in the rate-determining step. Rate data obtained in a temperature range of 750-1350 degrees C indicated a change of reaction mechanism at around 900 degrees C. At higher temperatures decomposition of adsorbed BCl3 to boron without the involvement of hydrogen should be considered. Decomposition or further reduction of the intermediate BHCl2 may also be significant at high temperatures.