O18 tracer is used to investigate the development of porous anodic films at constant current in phosphoric acid on electropolished aluminum. A barrier layer and porous region form initially with the pore size related to the surface texture of the substrate. Subsequently, major pores emerge, with their sizes related to the anodizing voltage. The evolution of the film is accompanied by increases in growth rate and formation efficiency. The O ions of a preformed oxide are retained in the film during anodization in a nonenriched electrolyte, with O18 being partitioned among (i) the surface region of texture-dependent porosity, (ii) the walls of major pores, and, in diminishing amounts, (iii) the inner region of the barrier layer.