The content of total dissolved solids and δD and δO18 values are given for 95 oil‐field brines from the Illinois, Michigan, and Alberta basins and the Gulf Coast. The variation in deuterium content among basins is found to be much greater than that within each basin. Oxygen isotopic composition, on the other hand, shows a large range in each basin, strongly correlated with salinity. The relationships between isotopic and chemical compositions of the brines lead to the following conclusions: (a) the water is predominantly of local meteoric origin; (b) the deuterium content has not been greatly altered by exchange or fractionation processes; (c) extensive oxygen exchange has taken place between water and reservoir rocks. Several samples contain water which appears to have originated as precipitation during Pleistocene glacial periods.

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