We investigated formation processes of a porous anodic alumina film on a p-type silicon (Si) substrate using infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). We observed drastic IR spectral changes when anodization took place near the interface between an aluminum (Al) layer and a Si substrate. The intensity of absorption peaks due to porous anodic alumina increased with a decrease in anodic current density and it decreased simultaneously with formation of silicon oxides (SiO2) at the interface between a porous anodic porous alumina film and a Si substrate after appearance of a spike of anodic current density which indicated changes of states of electric double layer at the interface between an electrolyte and an electrode due to contact between an electrolyte and a Si substrate. The results suggested that the formation of SiO2 nanodots on a Si substrate promoted penetration of electrolytes to peel the porous anodic alumina film from it. (C) 2004 Elsevier B.V. All rights reserved.