A NiO-MgO layered, binary thin oxide film has been prepared on a Mo(100) surface and characterized by Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS or EELS). NiO grows epitaxially on a 20-monolayer (ML) MgO(100) film supported on the Mo(100) surface. At a NiO coverage of < 1.8 monolayer, a new surface or interface state at 2.18 eV has been identified by electron energy loss. Furthermore, the Ni d-d transitions of the thin NiO film are also modified by the MgO substrate and shift gradually to the values observed for single crystal NiO(100) with increasing NiO coverage. The loss peaks at 3.03 eV (3A(2g) --> 3T(1g)) and 1.66 eV (3A(2g) --> 1E(g)) from a thick NiO film shift to 2.79 and 1.61 eV for a 0.5 ML NiO film, respectively, indicating a modification of the NiO electronic structure by the substrate MgO. A thick NiO film (> 15 hit) is thermally stable up to 1000 K, whereas interdiffusion of NiO with MgO occurs at the NiO/MgO interface at a considerably lower temperature (< 600 K).