Reaction products at inclusion sites on Al 2024 have been analyzed in the early stage of pitting by fluorescence microspectroscopy via functional near-field scanning optical microscopy (f-NSOM). From the invariant peak position on the f-NSOM fluorescence spectra at various reaction sites, it appears that the Al oxyhydroxide matrix surrounding the fluorescein dye species is uniform in composition and structure, inde pendent of location. Furthermore, changes in the peak intensity at these sites were proportional to the height of the product deposit. This means that the fluorescein is bound at sites in the product that are uniformly distributed. In addition, the number of binding sites is proportional to the thickness of the product. The chemical analysis of energy dispersive X-ray spectroscopy for the inclusion at each ring core revealed that both the height and intensity of the product rings around Mg-enriched Al-Cu-Mg inclusions were greater in value than, that around Mg-deficient inclusions and simultaneously enhanced with increasing width. This originates from the spontaneous At oxyhydroxide precipitation triggered by vigorous dealloying Mg ions from the large-sized inclusion with high initial content of Mg. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1859816] All rights reserved.