Lead blast furnace (LBF) slags are mainly composed of an iron-silica-lime glass matrix and minor phases such as solid solutions of Mg-Cr-rich and Fe-Zn-rich spinel crystals, crystallized iron oxides such as wustite and metallic lead droplets. In this study, results from Raman spectroscopy, transmission and scanning electron microscopy allow to argue that widely submicron iron-rich phases are very common in the glass matrix and could have an effect on the general alteration pattern of the glass matrix during leaching experiments. Open flow tests also point out close relationships between glass alteration ability and the presence of large lead droplets. According to numerous papers on nuclear glass leaching, acidic pH encountered in such open flow tests lead to preferential releases of the main alkali-earth metal composing the glass. Altered glass is mainly characterized by relative enrichments in iron. It is induced by high calcium or calcium-silicon depletions. Surprisingly, such type of alteration layer is not uniformly spread on the slag surface. In this work, it is also assumed that previous chemical analyses of the LBF glass matrix having micrometric resolution cannot give a realistic glass composition. A short discussion is also proposed about which characterization techniques can be used to correctly identify submicron iron-rich entities and evaluate their proportions and composition. (c) 2007 Elsevier B.V. All rights reserved.