The structure of an ordered, ultrathin theta-Al2O3 film grown on a NiAl(100) single-crystal surface was studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED), and its interaction with water was investigated with temperature programmed desorption (TPD) and XPS. Our results indicate that H2O adsorption on the theta-Al2O3/NiAl(100) surface is predominantly molecular rather than dissociative. For thetaH(2)O < 1 ML (ML = monolayer), H2O molecules were found to populate Al3+ cation sites to form isolated H2O species aligned in a row along the cation sites on the oxide surface with a repulsive interaction between them. For thetaH(2)O > 1 ML, three-dimensional ice multilayers were observed to form, which then desorb during TPD with approximate zero-order kinetics as expected. A small extent of H2O dissociation was observed to occur on the theta-Al2O3/NiAl(100) surface, which was attributed to the presence of a low concentration of oxygen atom vacancies. Titration of these defect sites with adsorbed H2O molecules revealed an estimated defect density of 0.05 ML for the theta-Al2O3/NiAl(100) system consistent with the ordered nature of the synthesized oxide film.