A number of marine water and fresh water samples were examined for the relative O18/O16 ratio, and the variation of this ratio was determined to a precision of ± 0.1%. In the case of surface marine waters, for a range of salinity of 29.40%., the O 16 content varies over a range of approximately 6%. The low O18/O16 ratios were obtained from surface marine waters contaminated with meltwater from the ice fields, while the marine waters of high salinity were richest in O 18. The observed relation between O 18 content and salinity of the oceanic waters can be explained by a process of multiple stage distillation which produces a continuous loss of fresh water to the ice regions from the surface waters of the warm oceans. The lower salinities of cold ocean currents, such as the Alaskan and Californian currents, are due primarily to mixing with meltwater from cold regions. The effect of glaciation upon the isotopic method of measuring paleotemperatures is discussed. The results for deep oceanic samples and for non-typical water samples are also discussed.