Aluminum was deposited on stoichiometric NiO(100) in 5 x 10(-6) Torr O-2 at substrate temperatures of 250 and 800 degrees C, and the resulting film interfaces were studied by photoemission [x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)I and electron diffraction [reflection high-energy electron diffraction (RHEED)]. At 800 degrees C, RHEED patterns indicate that the growing overlayer interacts with the NiO(100) substrate forming a NiAl2O4 spinel phase. This observation is further supported by XPS and UPS results where peak shapes and relative peak positions correspond to values reported for thick NiAl2O4 films. During film growth at 250 degrees C, the underlying NiO RHEED pattern becomes diffuse due to formation of an amorphous overlayer. XPS and UPS results indicate that the amorphous film is Al2O3 and that little interaction with the NiO substrate occurs. Postdeposition annealing of the amorphous Al2O3 film to 800 degrees C in O-2 results in a strong reaction at the interface and film crystallization to the NiAl2O4 spinel phase. These results demonstrate that kinetic factors are significant in determining interactions in this system.