Selected oil-field waters have been analyzed and studied from representative fields in New York, Pennsylvania, Ohio, and West Virginia. These waters, in general, occur in (1) the lower parts of folds and in synclines where they have been separated from the oil and gas by gravitation; (2) the lower parts of reservoir rocks underlying oil or gas “pays”; and, (3) closely associated and mixed with oil throughout the entire thickness of the reservoir rock. Concentration of the dissolved salts of the oil-field waters is usually much greater than present-day sea water and the relative amounts of the various radicals are changed. Increased concentration of the salts is attributed to the evaporative effect of expanding natural gas. The disproportionate concentration of the various radicles may be explained by applying the solubility product principle to sea water. Actual concentration of the water is well illustrated by studies of the flood waters of the Bradford field. In general, the waters occurring in Devonian rocks are more concentrated than those in Mississippian or Pennsylvanian rocks, suggesting that some migration and accumulation of oil may have occurred prior to the Appalachian revolution. Most Pennsylvanian waters are dilute solutions as compared with waters from older rocks, due probably to dilution by ground water and to limited migration and accumulation of oil in rocks of Pennsylvanian age.