Abstract The interactions of benzene, toluene, acetonitrile, pyrazine, and pyridine with hydroxylated ultrathin films of γ-Al 2O 3 were investigated using vibrational spectroscopy (high-resolution electron energy-loss spectroscopy). These molecules act as Lewis bases, forming acid/base complexes with the surface hydroxyl groups. Complex formation shifts the frequency of the isolated OH-stretch vibration (3711 cm −1 on the clean surface) to lower frequency. The magnitude of the frequency shift depends on the strength of interaction between the basic probe molecule and the surface OH groups. The strength of interaction is dependent on both the acidity of the surface OH groups and the basicity of the isolated probe molecule. Our data indicate that there are two distinct types of surface hydroxyl groups, which differ in surface acidity, on our thin films of hydroxylated γ-Al 2O 3 [J. Phys. Chem. B 107 (2003) 8538]. To determine the surface OH acidity for the surface hydroxyl groups on the hydroxylated γ-Al 2O 3, we followed the approach of Hair and Hertl [J. Phys. Chem. 74 (1970) 91] and developed a calibration plot using existing data available in the literature for silica, silica–alumina, magnesia, and phosphate on silica. Using this calibration plot, we determine a p K a=6.5±1.1 for the more acidic surface OH groups and a p K a=8.4±1.1 for the less acidic surface OH groups.