The present study employs a tungsten tracer incorporated into the aluminum substrate to investigate the development of porosity in anodic alumina formed in phosphoric acid electrolyte. An unusual inversion of the tracer distribution is revealed as the tracer layer traverses the barrier region. Although initially incorporated into the barrier layer at locations beneath pore bases, associated with the scalloped metal/oxide interface, the tracer at these locations subsequently lags behind that found at the cell walls. The behavior is contrary to expectations of a field-assisted dissolution model of pore development, with usual migration behaviors of film species in the barrier layer. However, the findings are consistent with pore formation due mainly to flow of alumina from the barrier layer toward the cell walls, driven by film growth stresses. Flow of film material can also account for the presence of phosphorus species in the film and the increased thickness of the film relative to that of the oxidized metal. (c) 2006 The Electrochemical Society.