Surface reactions between a (0 0 0 1) C-plane sapphire and NH3, with He as an inert carrier gas, were investigated at high temperatures over 1200 degrees C using the in situ gravimetric monitoring method. Although the sapphire substrate was stable up to 1400 degrees C under a He atmosphere, decomposition started to occur at 1300 degrees C under a 0.1 arm NH3+He and the decomposition rates were found to be lower than those in 0.1 atm H-2+He at each temperature. These results imply that sapphire can be decomposed by NH3 and/or hydrogen generated by the decomposition of NH3 over 1300 degrees C. The decomposition rate in NH3+He was decreased with increase in NH3 flow time, and the decomposition rate became constant after 60 min of NH3 flow. Moreover, the activation energy for sapphire decomposition before 60 min of NH3 flow was different from that after 60min of NH3 flow time, which indicates that the surface reaction between sapphire and NH3 and/or hydrogen generated from NH3 changes depending on the time of NH3 flow. The dependence of the surface reactions and rate-limiting reactions between sapphire and NH3 on the time of NH3 flow is discussed. (C) 2009 Elsevier B.V. All rights reserved.