The interfacial reactions induced by vacuum furnace annealing and rapid thermal annealing in sequentially deposited Al/Ni bimetallic thin-film diffusion couples have been investigated with MeV 4He+ backscattering spectrometry, cross-sectional transmission electron microscopy, and Auger electron spectroscopy. Upon annealing, NiAl3 is the first aluminide phase to grow. In uncontaminated samples, the NiAl3 growth proceeds in uniform planar fashion, governed by diffusion-limited kinetics. The kinetics data fit well with those for NiAl3 growth on large-grained Al substrates, yielding a common kinetics law of x2=kt, where x is the thickness of the NiAl3 grown at the interface, t is the annealing duration, and k is the growth constant, which is given by k=2.24(cm 2/s) exp(-1.5±0.1 eV/kBT), in which T is the annealing temperature and kB is the Boltzmann constant. Microscopic examination reveals slight nonuniformity at the Al/NiAl3 interface resulting from shallow local protrusions of NiAl3 grains into the Al layer at grain boundaries. When either the Al film or the Al/Ni interface is purposely contaminated during sample preparation, the roughness at this Al/NiAl3 interface becomes very pronounced, and the reaction rate is significantly reduced. Meanwhile, Ni motion becomes appreciable as NiAl 3 grains and/or Ni severely penetrate the Al layer. In contrast, the NiAl3/Ni interface remains sharp in all samples. The irregular morphology and nonuniform reaction cannot be attributed uniquely to the presence of grain boundaries in the Al film, but rather are a combined effect of impurities and Al grain boundaries. Short-term rapid thermal annealing at elevated temperatures appreciably alleviates the nonuniformity at the Al/NiAl3 interface in contaminated samples.