Oxygen and hydrogen isotope analyses were made on the carbonate-free fractions of twenty-seven ocean core samples from the Atlantic, Pacific, and Indian Oceans. δO18 values ranged from +11.5 to +28.5 permil, and δD values ranged from -5.5 to -8.7 per cent relative to SMOW. On the basis of chemical analyses the cores were divided into two groups, those with Fe + Mn between 7 and 15 wt. % and those with Fe + Mn between 2 and 5 wt. %. On the basis of mineralogic analyses by Goldberg and Griffin (1963, 1967) and isotopic and chemical analyses, the ferromanganese-rich samples appear to have a large authigenic component while the ferromanganese-poor samples appear to be primarily detrital. The oxygen isotopic compositions of the clay minerals in the detrital group of samples reflect the provenance and mode of origin of these minerals. There is no evidence for oxygen isotopic exchange, and inconclusive evidence for hydrogen isotopic exchange between these samples and sea water, even though some have been dated by the ionium-thorium method as being as old as 250,000 years (Goldberg and Griffin, 1963). The range of published δO18 values of shales, supplemented by two new analyses, is similar to that of the ocean core samples of detrital origin. Therefore, as far as the isotopic record is concerned it is impossible to distinguish between marine and non-marine shales. Material balance calculations suggest that the effect of weathering and sedimentary processes has been to deplete the hydrosphere of O 18 by about 3 permil and to enrich the hydrosphere in deuterium by about 0.03 per cent during the earth's history.