We have investigated the alteration behavior of synthetic basalt and SRL 165borosilicate waste glasses that had been reacted in water vapor at 70°C fortime periods up to seven years. The nature and extent of corrosion ofglasses have been determined by characterizing the reacted glass surfacewith optical microscopy, scanning electron microscopy (SEM), transmissionelectron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS).Alteration in 70°C laboratory tests was compared to that which occurs at150–200°C and also with Hawaiian basaltic glasses of 480 to 750 year oldsubaerially altered in nature. Synthetic basalt and waste glasses, bothcontaining about 50 wt % SiO2, were found to react with watervapor to form an amorphous hydrated gel that contained small amounts ofclay, nearly identical to palagonite layers formed on naturally alteredbasaltic glass. This result implies that the corrosion reaction in naturecan be simulated with a vapor hydration test. These tests also provide ameans for measuring the corrosion kinetics, which are difficult to determineby studying natural samples because alteration layers have often spalled offthe samples and we have only limited knowledge of the conditions under whichalteration occurred.