<p>Corrosion mechanism of minerals and glass is a critical study domain in geology and materials science, vital for comprehending material durability under various environmental conditions. Despite decades of extensive study, a core aspect of these mechanisms—specifically, the formation of amorphous alteration layers upon exposure to aqueous environments—remains controversial. In this study, the corrosion behavior of a boro-alumino-phospho-silicate glass (BAPS) was investigated using advanced solid-state nuclear magnetic resonance (SSNMR) and SEM techniques. The results are consistent with a uniform nanoscale phase separation into aluminum phosphrous -rich and aluminum silicate-rich domains. During corrosion, the aluminum phosphrous -rich domains undergo gelation, whereas the aluminum silciate-rich domains remain vitreous, forming a gel layer comprised of both phases. Although SEM images show a sharp gel/glass interface—suggestive of a dissolution-precipitation mechanism, multiple lines of evidence indicate that an in situ transformation mechanism is more consistent with our findings, even if dissolution–precipitation cannot be entirely excluded. This in situ transformation is governed by a series of coupled chemical reactions, involving: (i) preferential hydrolysis of aluminum phosphrous -rich domains leading to porous gel regions; (ii) retention of aluminum silciate-rich glass domains within the gel layer, with water infiltrating inter-network spaces; and (iii) selective leaching of phosphorus over aluminum, leading to reorganization of the gel network.</p><p></p>