Ultrathin NiO films have been prepared on an Al2O3(0001) surface, supported on a Mo(110) or Re(0001) substrate. The electronic and chemical properties of these NiO films have been studied using electron energy loss spectroscopy (ELS), temperature programmed desorption (TPD), and infrared reflection adsorption spectroscopy (IRAS). TPD and IRAS of adsorbed CO show that the chemical properties of the thin (< 10 monolayers) NiO films are considerably different from those of thick (> 10 monolayers) NiO films. The electronic structure, however, for NiO films with thicknesses between 0.5 and 28 ML are indistinguishable by ELS. Heating monolayer NiO on Al2O3 to 800 K in vacuum leads to a substantial reduction of NiO to metallic Ni.