The interaction of water vapor with amorphous aluminum oxide films on Al(111) is studied using X-ray photoelectron spectroscopy to elucidate the passivation mechanism of the oxidized Al(111) surfaces. Exposure of the aluminum oxide film to water vapor results in self-limiting Al2O3/Al(OH)(3) bilayer film growth via counter-diffusion of both ions, Al outward and OH inward, where a thinner starting aluminum oxide film is more reactive toward H2O dissociation-induced oxide growth because of the thickness-dependent ionic transport in the aluminum oxide film. The aluminum oxide film exhibits reactivity toward H2O dissociation in both low-vapor pressure [p(H2O) = 1 x 10(-6) Tord and intermediate-vapor pressure [p(H2O) = 5 Torr] regimes. Compared to the oxide film growth by exposure to a p(H2O) of 1 x 10(-6) Torr, the exposure to a p(H2O) of 5 Ton results in the formation of a more open structure of the inner Al(OH)(3) layer and a more compact outer Al2O3 layer, demonstrating the vapor-pressure-dependent atomic structure in the passivating layer.