The structure and surface acidity of thin water-oxidized alumina films on Al(110) have been studied as a function of film treatment temperature and ethylene adsorption using X-ray photoelectron spectroscopy. The OH content of the oxides decreases monotonically with temperature while the crystallinity increases uniformly to form a polycrystalline gamma-alumina at 500 degrees C. Ethylene adsorbed on the oxides identifies the presence of acid sites with a maximum spectral intensity for alumina films pretreated between 350 and 400 degrees C. An interpretation is proposed in terms of a molecular orbital adsorbate-surface model based on the core level and valence level photoemission data to explain how chemisorbed molecular ethylene adsorbate may form from a a-complex adduct on the acid sites. The results are consistent with the formation of a (C2H5)-C-+ carbenium ion.