Aluminum foils with two different surface topographic textures were anodically oxidized at constant current in a phosphoric acid bath. In situ atomic force microscopy (AFM) was used to follow the initial development of surface topography on a 1 mu m scale, during the early stages of porous oxide film formation. Microscopic convex features such as ridges on both foils begin to increase in height and width when the anodic film thickness exceeds the initial feature height. Equations of a mathematical model are presented incorporating established interfacial reactions and oxide conduction behavior. The model indicates that the film-solution interface recedes into the metal during anodizing, since the current efficiency for oxide formation is smaller than the oxygen ion transport number in the film. Ridge surfaces increase in height due to the higher local conduction resistance to the film-solution interface, while film deposits rapidly at ridges because of the low local resistance to the metal-film interface. In agreement with the AFM results, enhanced oxide growth at ridges should start when the potential field in the film becomes two-dimensional, as a result of the film growing to a thickness larger than the ridge height.