Theoretically, the chlorine isotope ratio, ³⁷Cl/³⁵Cl, can aid in identifying chloride sources and in distinguishing chloride-transport mechanisms among formation waters if ratio differences exist among samples. We measured the chlorine ratios of 18 formation-water samples from oil fields along the Texas-Louisiana Gulf Coast representing depths between 2,016 and 4,267 m. Chloride concentrations of samples ranged from 4 to 94.874 g/L, the chlorine ratios ranged from -1.24 to 0.58 ^pmil (with respect to standard mean ocean chloride or SMOC, precision 0.12 ^pmil and enrichment of ³⁷Cl correlates with increasing concentration. We also compared the isotope ratios of samples from within the Weeks Island salt dome with adjacent formation waters in Louisiana. All ratio measurements on the salt are between 0.1 and 0.3 ^pmil, and for the formation waters are between 0.0 and -0.22 ^pmil. The probability that the salt measurements differ from the waters is > 99%. We conclude that significant chlorine isotope ratio differences exist among formation waters and between the waters and potential sources in the Gulf Coast region, and that the chlorine ratio will be a valuable tool in interpreting chloride geochemistry in this region.