Pyridine adsorption on NiAl(100) and ultrathin films of gamma-Al2O3 was studied using high-resolution electron energy loss spectroscopy (HREELS). Pyridine adsorbs on NiAl(100) with its molecular axis parallel to the surface plane at low coverages and with its molecular plane inclined more toward the surface normal at higher coverages. On the hydroxylated and nonhydroxylated thin films of gamma-Al2O3, pyridine interacts with the coordinatively unsaturated Al3+ cations via its nitrogen lone-pair electrons. Pyridine was also found to interact with the surface hydroxyl groups on the hydroxylated gamma-Al2O3 thin films, forming C5H5N-HO complexes. Complex formation causes the OH bond strength to decrease and the OH stretch vibration to shift from 3711 cm(-1), which is characteristic of uncomplexed, isolated OH, to lower frequency. At low coverages, pyridine only interacts with the more acidic surface OH groups, those located on 3-fold Al3+ cation sites. This interaction forms a C5H5N-HO complex, which has an O-H stretch at 2920 cm(-1). As the coverage is increased, an additional C5H5N-HO complex is formed from an interaction between pyridine and the less acidic OH groups, which are bonded to 2 Al3+ cations. The vibrational frequency for O-H stretch of this C5H5N-HO complex is 3150 cm(-1). As the intensities for the O-H stretches of the C5H5N-HO complexes increase, the intensity for the free O-H stretch at 3711 cm(-1) decreases. The interaction with the surface hydroxyl groups is reversible, confirming that the observed shifts in the O-H stretching frequency result from the formation of weakly bonded acid-base complexes. While most of the pyridine desorbs from the gamma-Al2O3 thin films by 290 K, annealing the pyridine-dosed gamma-Al2O3 thin films to temperatures above 290 K, results in a small amount of pyridine dehydrogenation and the formation of surface OH groups with an O-H stretch at 3742 cm(-1).