Electron‐beam‐evaporated aluminum/silicon oxide/silicon tunnel diodes with an initial oxide thickness of 1.3 nm have been annealed for up to 1 h at temperatures from 213 to 369 °C. They have been investigated by infrared grazing internal reflection (GIR) spectroscopy and current‐voltage measurements. The measured IR spectra were analyzed by computer modeling. All spectral features could be explained self‐consistently within a Al/AlOy/SiOx/Si layer model. In the as‐deposited state less than 0.6 monolayers of Al—O bonds are formed at the Al/SiOxinterface. A thermally activated reduction of the ultrathin oxide film by Al was observed. The changes in the current‐voltage curves induced by slight annealing (1 min at 213 °C) are accompanied by changes in the insulator‐bonding structure, which GIR is sensitive enough to detect.