EIS in the 100 kHz-1 mHz frequency range was applied to the study of changes in cold sealed aluminium oxide films exposed to highly wet and extremely dry atmospheres. Information about these changes was obtained from evolution of the impedance diagrams and, in particular, from film resistance and capacitance values determined from them. Results show that sealing quality, assessed from EIS, increases over months and years as ageing proceeds in a natural atmosphere. The analysis was completed with the aid of XPS and EDX techniques and standard quality control tests. Measurements show that specimens aged very rapidly in wet atmospheres, so that they passed all sealing quality tests within 72 h. The sealing quality improves with ageing even in highly dry atmospheres despite the fact that pores lose part of their initial filling water. XPS analysis revealed that fluorine and nickel concentrate in the outer layers of cold sealed anodic films; on the other hand, films obtained by the traditional HTS procedure exhibit a uniform composition throughout their thickness.