Experimental alteration of obsidian was performed in distilled-deionized water at 100°C for up to 365 days to elucidate formation processes of clay minerals at relatively low temperature condition. The alteration products were examined by transmission electron microscopy (TEM), scanning electron microscopy, and energy dispersive X-ray analysis. The surface compositions of obsidian before and after alteration were also examined by X-ray photoelectron spectroscopy (XPS) to clarify dissolution processes. For reaction products, allophane, fibrous boehmite, and halloysite were formed at early stage while fibrous smectite appeared later. Based on the activity diagram for the system of Na₂ OAl₂O₃-SiO₂-H₂O, it was found that allophane, boehmite, and halloysite appeared to be formed as a metastable phase, and that smectite was produced as a stable phase. TEM showed that allophane and boehmite were precipitated from the solution, and halloysite and smectite were then transformed from allophane and boehmite, respectively, as reaction proceeded. XPS indicated that preferential leaching of Na and enrichment of K occurred at the obsidian surface. Likewise, Al/Si ratio of the surface increased during the reaction. These results are probably due to formation of smectite structure in a leached layer produced on the obsidian surface.