The effect of an aluminum oxide layer at the interface of Al/Nb on phase formation in thin‐film binary diffusion couples was studied. Al/Nb bilayers with or without aluminum oxide at the interface were prepared by electron‐beam evaporation. The samples were annealed at a constant heating rate of 50 °C/min to temperatures in the range 800–950 °C, and the progress of the reaction was monitored by x‐ray diffraction and measurement of the superconducting transition temperature. The presence of an aluminum oxide layer at the interface was found to significantly retard the formation of the superconductingA15 Nb3Al phase.