Hydroxylated thin films of @gamma@-Al@sub 2@O@sub 3@ are grown by exposing a NiAl(100) single crystal to 100 L H@sub 2@O at 1000 K. In HREELS analysis, these films exhibit the phonon modes in agreement with @gamma@-Al@sub 2@O@sub 3@ films grown on the NiA l(100) substrate using 2400 L O@sub 2@ at 1000 K. In addition to the observed phonon modes, a single OH stretch is observed at 3270 cm@super -1@ (FWHM approximately 100 cm@super -1@). This frequency is indicative of non-interacting OH groups bonded to 2 or 3 Al atoms. To characterize the acidity of the surface OH groups, we have studied the adsorption of probe molecules, such as pyridine, benzene, and toluene, on the hydroxylated @gamma@-Al@sub 2@O@sub 3@ surface at 150 K. The probe molecules, acting as bases, form acid-base complexes with the surface OH groups. HREELS spectra were recorded as a function of probe molecule coverage. Complex formation causes the OH bond strength to decrease and the OH stretch to shift to lower frequency. The adsorption and complex formation is observed to be reversible. The OH frequency shift is dependent on the probe molecule basicity and the OH acidity. Our results indicate that two types of isolated OH with different acidity exist on the surface. The correlation between OH frequency shift and probe molecule basicity allows us to determine the acid strength of the surface OH groups quantitatively.