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Experiments on glassy rhyolite and phenocrystic andesite-seawater interactions were carried out at 300°C and 1,000b using Dickson-type gold bag equipments. Chemical reactions in rhyolite-seawater inter action proceeded very rapidly and a steady state was attained within about 48h. Andesite-seawater inter action, however, proceeded very slowly and a steady state was not attained within the run period of 7,090h. Rhyolite and glass, augite and hypersthene in andesite were altered into smectite. Seawater lost most Mg2+ and S042 and become acid Na-K-Ca-Cl-type solution through the reactions.