Sequential anodizing is employed to investigate the mechanism of coating growth on aluminum under sparking in a phosphate electrolyte. Thus, coatings are formed in a silicate electrolyte and then in the phosphate electrolyte, with the location of phosphorus in the final coating being identified by electron probe microanalysis. The presence of silicon primarily in the outer region of the coating and of phosphorus primarily in the inner region of the coating indicates short-circuit transport of phosphorus species through the coating, with new coating material being formed in the vicinity of the metal/coating interface. The transport path is presumed to be associated with pores, cracks, and channels in the coating produced by dielectric breakdown events. © 2005 The Electrochemical Society. All rights reserved.