In this study, transmission FT-IR spectroscopy is used to investigate the adsorption of water on aluminum oxide surfaces, including both single-crystal and particle surfaces. The FT-IR spectra of the (0001) surface of alpha-Al2O3 single crystals at 296 K in the presence of 0.2 to 20 Torr H2O vapor pressure corresponding to 1 to 95% relative humidity (RH) have been measured. The FT-IR spectra are shown to change as a function of RH. At low relative humidity, below approximately 10%, water adsorbs on the surface in an ordered fashion with the formation of a stable hydroxide layer. At intermediate RH, between 10 and 70% RH, water adsorbs molecularly in a structured overlayer. At high RH, greater than 70%, water adsorption is more disordered and consistent with the formation of a liquid-like water layer. The adsorption of nitric acid on alpha-Al2O3(0001) prior to water adsorption is shown to only slightly modify the water infrared spectra. A discussion of the quantification of the number of water layers on the alpha-Al2O3(0001) surface as a function of RH as well as a comparison of water adsorption on single-crystal alpha-Al2O3(0001) to that on a- and gamma-Al2O3 powders are presented here.