As part of the investigations on the suitability of a new concept for a tailored fiber-matrix interface in sapphire fiber reinforced NiAl matrix composites for application as a high-temperature structural material, the interfacial reactions in the system alumina-hexagonal boron nitride-yttrium (Al2O3-hBN-Y) have been examined in the temperature range of 1100-1300 degreesC. For this, alumina substrates were coated with hBN by means of CVD and subsequently with sputter deposited yttrium. Afterwards the samples were annealed for up to 16 h under inert atmosphere. Grazing incidence X-ray diffraction (GIXRD) served to analyze the phases formed by diffusion processes in the reaction zone. The peak intensities in these diffraction patterns were used to evaluate the sequence of phases formed due to diffusion and reaction. After the initial formation of YN and YB2, the phases Y2O3, Al2Y, and YB4 were observed. Even longer annealing times or higher temperatures, respectively, led to the formation of the ternary oxides YAlO3 and Y3Al5O12 as well as metallic aluminum.