Abstract Explosive volcanic eruptions frequently transport large amounts of silica-rich tephra glass to the deep sea. Some studies of the geochemistry of distal volcanoes are based on the assumption that no chemical exchange has taken place between the glass and adjacent pore water. Most models for silicate glass and liquids suggest that water must travel through the glass as a dissociated molecule, and hence any increase in water content must be accompanied by exchange of other ions to maintain charge balance. Studies of newly erupted tephra glass indicate low water contents of 0.3 to 1.0 wt%. Analysis of tephra glass from piston cores and Deep Sea Drilling Project cores reveal that water uptake is initially rapid, increasing to 4 wt% in 100 000 y. Any additional water uptake takes place very slowly over the next several million years. Abyssal tephra glasses with water contents over 6 wt% are extremely rare. While the oldest glasses show solution pits and etching features, the glass composition as determined by electron-probe microanalyzer does not show any indication of chemical exchange. Traverses across individual shards fail to show any major-element heterogenity or indication of diffusion gradients.