An investigation was carried out on the composite of Ni 3 Al reinforced by alumina-silica fibers (Sumitomo fiber). Electron microscopy (TEM, SEM) and microanalysis (XEDS, EELS and electron diffraction) were employed to study the microstructure development and the chemical stability of the components and their interfaces during the diffusion bonding (1200-1300 o C/10-30 MPa/2 h) in vacuum. Two different types of chemical reaction occurred in the fiber during hot pressing. The reaction between the fiber components, nanocrystalline γ-Al 2 O 3 and amorphous SiO 2 (intrinsic reaction), generated mullite (Al 6 Si 2 O 1 3 ). No crystallographic texture developed in the newly formed mullite grains. Other reactions (extrinsic reactions) were the formation of nickel oxide particles in the fiber and long shaped α-Al 2 O 3 in the interface. The former occurred by a chemical reaction of the diffusing Ni and free oxygen in the fiber, and the latter was confirmed to be the product of the reaction between free oxygen and Ni 3 Al in the interface. The required free oxygen for the extrinsic reactions was inferred to stem from the reduction of silica at the hot pressing temperatures.