The Interfacial reactions between Al and RuO2, MoOx and WNx diffusion barriers on Si (100) wafers have been studied. The diffusion barrier structures were analyzed before and after various heat treatments using Auger electron spectroscopy (AES) and cross‐sectional transmission electron microscopy (XTEM). Al was found to reduce the oxides of both Ru and Mo. A 100 Å thick Al2O3 layer developed between the Al and the RuO2 films during annealing at 500°C for 30 minutes. The formation of interfacial Al2O3 efficiently prevents Al penetration but may cause a too high contact resistance for applications in contact structures of microelectronic devices. The WNx barrier was stable after annealing at 600°C for 30 minutes, if the film was exposed to air prior to Al deposition. If, instead, the Al was deposited in situ, the structure failed after annealing at 500°C due to a reaction between Al and W giving WAl12. A thin Al2O3 layer was detected between the Al and the WNx films of the air exposed sample. This layer had the effect of retarding the Al–W reaction and thereby raised the failure temperature of the barrier by at least 100°C. Copyright © 1989 John Wiley & Sons Ltd.