Abstract: Experimental alteration of obsidian in distilled-deionized water at 150°, 175°, 200°, and 225°C was studied. The alteration products were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX) to evaluate the formation process of clay minerals. The surface composition of obsidian before and after alteration was examined by X-ray photoelectron spectroscopy (XPS), and concentrations of released elements in solution were measured to elucidate alteration and dissolution processes. TEM clearly showed that allophane appeared as the first reaction product in each experiment. With increasing reaction length, noncrystalline straight fibrous material was formed in the aggregates of allophane particles as a metastable transitional phase, and tended to form curled or wavy bundles of fibers with longer reaction. The non-crystalline fibers were transformed into highly curled smectite exhibiting small circular forms less than 1.0 µm in diameter as reaction progressed. EDX confirmed that the smectite consisted mainly of Si, Al, and small amounts of Ca, K, and Fe. XPS revealed the formation of a dealkalized leached layer on the surface of obsidian during the reaction. The concentration of released elements suggested that nonstoichiometric dissolution proceeded during the reaction.