The coadsorption of H2O and NO2 molecules on a well-ordered, ultrathin theta-Al2O3/NiAl(100) film surface was studied using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS). For H2O and NO2 monolayers adsorbed separately on the theta-Al2O3/NiAl(100) surface, adsorption energies were estimated to be 44.8 and 36.6 kJ/mol, respectively. Coadsorption systems prepared by sequential deposition of NO2 and H2O revealed the existence of coverage and temperature-dependent adsorption regimes where H2O molecules and the surface NOx species (NO2/N(2)O(4)f/NO2-,NO3-) form segregated and/or mixed domains. Influence of the changes in the crystallinity of solid water (amorphous vs crystalline) on the coadsorption properties of the NO2/H2O/theta-Al2O3/NiAl(100) system is also discussed.