Hard anodic layers produced under DC conditions in sulphuric acid on a commercial AA 6060 aluminium alloy have been characterised by electron microscopy and secondary ion mass spectroscopy (SIMS). Once exposed during anodising, alpha-Al(Fe, Mn)Si intermetallic particles start to corrode in the electrolyte. Partially corroded particles become trapped in the oxide, disturbing the local pore structure. This leads to pyramidal defects on the alloy surface and retention of isolated, unmodified fragments of alloy in the oxide. Sealing in boiling water left a low density, loosely attached surface layer of modified oxide that gave poor adhesion properties. The bulk oxide pore structure was modified by a uniform, fine network of bridges that extended down the barrier layer. Calibration of the SIMS sputtering rate allowed the concentration of alloying and impurity elements to be measured as a function of depth in the oxide and into the alloy.