Although internal stress is frequently being suggested as a plausible reason for oxide breakdown during valve metal anodising. no direct quantitative evidence has been made available yet. In this work, we anodized sputtered zirconium thin films galvanostatically at room temperature in sulphuric acid until breakdown was observed, and simultaneously measured the internal stress evolution in the oxide in situ, using a high-resolution curvature setup It was found that the higher the magnitude of the observed internal compressive stress in the oxide, the smaller the oxide thickness at which breakdown occurred The moment of breakdown was identified from a slope change in the cell voltage evolution, indicative fora decrease in anodising efficiency The latter presumably occurs as a result of oxygen evolution, initiated by the relative increase of the cubic or tetragonal zirconia phase content relative to the monoclinic one. This was evidenced in turn by comparing electron diffractograms. taken in a transmission electron microscope, before and after breakdown The critical role of internal stress on oxide breakdown during zirconium anodising can therefore be associated with its promoting effect on the densifying phase transformation of monoclinic oxide. (C) 2010 Elsevier Ltd. All rights reserved