The stability of the NiAl/MgO interface at temperatures in the range 800-1000 degrees C was studied on layered thin films of MgO/(Ni + AI)/MgO, using electron diffraction in transmission electron microscopy, and X-ray photoelectron spectroscopy. NiAl was formed by rapid thermal annealing (RTA) of the films at 300 degrees C, without affecting the interface. RTA at 800 degrees C, for 5 min induced a limited interfacial reaction which formed a spinel phase and Ni3Al, but left most of the NiAl and MgO layers intact. In the earliest reaction stages, aluminium diffuses out from, and oxygen diffuses into NiAl. After RTA at 1000 degrees C for 100s, the NiAl layer disintegrates completely, while the magnesium apparently evaporates from the MgO. While the NiAl/MgO system is shown to be chemically incompatible on the nanoscale, comparison with other aluminide/reinforcement systems shows it to be one of the more stable ones.