The growth of an anodic oxide film on a specimen consisting of a layer of aluminum deposited upon a layer of tungsten was examined by transmission electron microscopy. The findings reveal initial growth of anodic alumina, followed by penetration of the alumina by fingers of tungsten oxide once the metal/film interface reaches the tungsten layer. The fingers are approximately cylindrical or lanceolate, with width in the range 1-15 nm and aspect ratios up to about 40. The current flows preferentially to regions of tungsten oxide rather than the alumina due to the lower electric field for growth of the former oxide. The penetration of the alumina is assisted by the relatively high Pilling-Bedworth ratio for W/WO3 and the inherent plasticity in amorphous anodic oxides. The tungsten oxide of the fingers appears to be of higher density than that formed at the metal/film interface after the aluminum has been fully oxidized, which may be due to the penetration of the alumina layer by a displacement process.