Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72 °C, composed of sodic and non-sodic glasses of surface activity caused by IVAl substituting in Q4(1Al) and Q4(2Al) positions, minor VAl, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H2Om in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H2Om molecules. Sodic glasses contain 71.80–77.77 wt% SiO2, are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 μm size). Sodium-free glasses are more siliceous, with 74.84–83.88 wt% SiO2, show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 μm size).