Porous alumina films have been synthesised by anodisation of high purity aluminium in 0.4 M phosphoric acid at constant current density. The dissolution process of those coatings in 2 M phosphoric acid has been investigated by in situ spectroscopic ellipsometry (SE) combined with electrochemical measurements. A sharp drop of the open current potential is observed due to film removal. The dissolution time depends on total barrier layer thickness. Those results are confirmed by SE measurements. A three layer optical model has been used to interpret SE spectra. During the dissolution process. the porous top layer has a quasi-constant thickness. whereas porosity increases significantly. The dissolution proceeds within the pores. Only the thickness of pure alumina barrier layer (upper part) decreases during the dissolution process whereas the interface barrier layer (lower part) does not change.