Abstract Transmission electron microscopy of ultramicrotomed sections of anodic barrier-type films formed on aluminum in tungstate solution reveals directly the duplex nature of the anodic films. Immediately on exposure to the electron beam, a relatively dark region of film material is evident adjacent to the film-solution interface and separated from the metal substrate by a more extensive lighter region of film material. Subsequent investigation using a combination of scanning transmission electron microscopy (STEM) and energy-dispersive analysis of X-rays (EDAX) confirms the presence of tungsten in the outer darker region of film material, and induced crystallization shows that the inner region of relatively pure alumina crystallizes at a faster rate than the region in which tungstate anions are incorporated. Thus, depending on the anodizing conditions, three techniques are available for the determination of the extent of such anion incorporation readily in the electron microscope, which give further insight into the apparent transport numbers during film growth. For the situation in which anodizing is carried out in a solution containing an anion of a heavy metal with a relatively high electron-scattering cross section, and in which such species are incorporated within the film, apparent transport numbers can be determined directly without the need for induced crystallization and/or STEM-EDAX analysis.