The 18O/16O ratios of fresh unmetamorphosed basalts from the Reykjanes, Mid-Atlantic, and Gorda Ridges and the East Pacific Rise fall in a narrow range of 5.5–5.9‰ (SMOW), identical to those of basalts from oceanic islands. Large plagioclase xenocrysts in some of the basalts are not in isotopic equilibrium with coexisting olivine. The basalts react with sea water at the ambient sea-floor temperature to form clays, resulting in an increase in δ18O of the basalt of 0.25‰ per m.y. This provides a very sensitive method to detect early stages of weathering, which otherwise might go unnoticed. Advanced submarine weathering of basalt can remove nearly one half of its SiO2, MgO, CaO, and Na2O and lesser amounts of Al2Os and TiO2. Weathering does not proceed deeply enough into the basaltic sea floor to affect the chemical composition of sea water. Since it is likely that large amounts of basaltic debris (less than a few cm in radius) are on the ocean floor, their weathering can affect the ocean-sediment budget of TiO2, Al2O3, SiO2, MgO, and CaO.