Reaction kinetics between borosilicate glass (BSG) and sapphire has been studied at temperatures from 850-degrees-C to 950-degrees-C. Microstructural and chemical analyses show that the nonporous interdiffusion layer is formed with Al+3 ion dissolving from sapphire and K+ diffusing from BSG onto the interface of sapphire/BSG, and that both ions are always coupled together in the reaction layer. The interdiffusion layer moves toward BSG with time and the reaction starts immediately at temperatures investigated without incubation period. The growth kinetics for the interdiffusion layer follows a parabolic rate law in the temperature range investigated, and shows an apparent activation energy in the range of 176 kJ/mol. The diffusion coefficient of aluminum ion is determined from EDX analysis, and the values range from 0.7-1.4 X 10(-12) at 850-degrees-C to 3.0-6.0 x 10(-12) cm2/s at 950-degrees-C. The above results also show an activation energy close to that determined from the parabolic growth rate constants, suggesting that the mass-transport kinetics of aluminum ion from sapphire into the interdiffusion layer controls the formation process.