To investigate the reduction of NiAl2O4 and the sintering of Ni on alpha-Al2O3, Ni layers have been deposited onto polycrystalline alpha-Al2O3 substrates. Some samples were oxidized at 700 degrees C and annealed at 1100 degrees C in N-2/O-2 to convert the nickel layers completely into NiAl2O4. The reduction and sintering behavior of these layers was studied by Rutherford backscattering spectrometry, scanning electron microscopy, and X-ray diffraction. Temperature-programmed reduction showed that the onset temperature for the reduction of bulk NiAl2O4 is about 870 degrees C. Due to the high temperatures required for the reduction of the NiAl2O4/Al2O3 samples, sintering of Wi during this treatment was inevitable. Our results indicate that there is a critical transition temperature for Ni sintering between 450 and 500 degrees C. Nickel layers deposited onto NiAl2O4/Al2O3 samples showed exactly the same sintering behavior as Ni layers on bare alpha-Al2O3; thus, a continuous interfacial layer of NiAl2O4 does not inhibit sintering. However, substantially less sintering was observed for Ni/NiAl2O4/Al2O3 samples containing Ni particles on top of NiAl2O4 islands. A discontinuous interfacial NiAl2O4 layer is apparently required to slow the sintering of nickel on alpha-Al2O3.