Chemical dissolution of the barrier layer of alumina films formed on 98% aluminium in the 4% oxalic acid after immersion in 2 mol dm(-3) sulphuric acid at 50 degreesC have been studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. Such defects as micropores, which reach the aluminium basis in the barrier oxide layer, were detected after the beginning of dissolution. It was established that the barrier layer material might consist of two or three regions according to the dissolution rate. The barrier oxide contains two layers at 50 V: the outer layer with the highest dissolution rate and the inner layer with low dissolution rate. The barrier oxide contains three layers at 60 V and above it: the outer layer with high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with low dissolution rate. A higher anodizing voltage leads to a higher dissolution rate of the outer and middle layers. We suggest that the change in the amount of layers in the barrier oxide is linked with the change in the growth mechanism of porous alumina in oxalic acid above 55 V (C) 2004 Elsevier B.V. All rights reserved.