Abstract: Municipal solid waste incinerator (MSWI) bottom ash is the slag-like material produced by the incineration of municipal waste and is predominantly composed of glassy constituents, which include inherited manufactured glasses and glasses formed during incineration. Previous results indicate evidence of neoformation of well-ordered clay (illite) from glasses in MSWI bottom ash after 12 y of natural weathering. We investigated the mechanism and conditions of clay formation from glasses during natural weathering using transmission electron microscopy (TEM), experimental leaching experiments and ammonium oxalate extractions. It was concluded that the high surface area and initially high “active” Al and associated Si content predispose the ash to form clay minerals on a relatively short time scale. This work provides evidence that the composition of secondary amorphous aluminosilicate and thus, the type of clay mineral which may form, is determined by the pH of the pore solution rather than by the glass composition. Presumably alternate wetting and drying of the ash during disposal greatly accelerates the formation of well-ordered clays.