The nature of freshly prepared and hydrothermally treated anodic alumina films has been studied by infra-red spectroscopy supported by electron microscopy. A comparison of the spectra obtained from freshly prepared specimens and those obtained after evacuation and deuterium exchange respectively, suggests that unsealed films are a relatively open array consisting of amorphous alumina crystallites and are permeated by molecular water, the surfaces of the crystallites carrying hydroxyl groups or ions. Hydrothermal treatment causes the structure to re-arrange by an agglomeration process into a more closely packed configuration, limiting the removal of molecular water and the extent of deuterium exchange, eventually blocking the major pores and also possibly progressively increasing the hydroxyl content of the film. By correlation of these results with visual and structural evidence, a relatively detailed morphological, structural, and analytical model is proposed for both the “unsealed” and “sealed” film.