The products from experimental alteration of crystalline and glassy basalt by meteoric water at ~45 and ~70°C have been analyzed with transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray analytical electron microscopy (AEM). The principal solid alteration products include quartz, chrysotile, talc, kerolite, calcite, kaolinite, smectite, and amorphous silicate material with spongy, smectite-like morphology. These solids act as sinks for elements that are released during the basalt alteration, and they are important for controlling the chemistry of the altering fluid. Comparison of the reaction quotients for the various solid phases and the fluid composition shows that the approach to equilibrium between altering fluid and basalt depends on both temperature and the ratio of fluid to basalt surface area. The degree of supersaturation with respect to equilibrium values for the precipitated phases can be explained by the enhanced solubility resulting from small crystal size of the alteration products. The results of this study underscore the importance of metastabile phenomena in the low-temperature alteration or weathering of both crystalline and glassy basalt.