The Mahoning River drains the densely populated and industrialized Warren-Youngstown area in northeastern Ohio. Significant chemical constituents and physical properties generally regarded as important in establishing water-quality standards for the Mahoning River are evaluated on the basis of hydrologic conditions and water use. Most of the interpretations and the appraisal of water-quality conditions are based on data collected from January 1963 to December 1965. Generally, streamflow during this period was lower than during a selected long-term reference period ; however, extremely low flows that occurred in the reference period did not occur in the 3-year study period. Water temperatures of the Mahoning River at Pricetown and Leavittsburg were not affected by thermal loading. Water temperatures at those stations ranged from the freezing point to 78?F during the 1963-65 period. Downstream from Leavittsburg, the use of large quantities of water for industrial cooling caused critical thermal loading during periods of low streamflow. Maximum water temperatures were 108?F and 104?F at Struthers and Lowellville, respectively. Water temperatures of the Mahoning River were lower during high water discharges and increased with higher steel-production indices. Flow augmentation and modifications in industrial processes have improved the water-temperature conditions in recent years. A combination of oxygen-consuming materials and warmed water from industrial and municipal wastes discharged into the lower reaches of the Mahoning River frequently depleted the dissolved-oxygen content. At Lowellville, the river water had a dissolved-oxygen content of 5 ppm (parts per million) or less for 67 percent of the time and 3 ppm or less for 16 percent of the time during the study period. The percentage of saturation of dissolved oxygen followed a similar trend. Both the dissolved-oxygen concentration and the percentage of saturation were noticeably lower downstream from Leavittsburg during the warm months when water temperatures were high and streamflow was low. The dissolved-oxygen content in the Mahoning River at Leavittsburg and Pricetown was almost always at acceptable levels. The calculated dissolved-solids concentration of the Mahoning River ranged from 150 to 450 ppm at Leavittsburg and from 200 ppm to 650 ppm at Lowellville. Industrial use of the water caused an increase in the dissolved-solids concentration at Lowellville. During one steel-mill shutdown the average dissolved-solids concentration decreased from about 360 to about 280 ppm. Chloride concentrations in the Mahoning River ranged from 42 ppm at Pricetown to 108 ppm at Struthers. The chloride load at 50-percent flow duration was 9 and 69 tons per day at Pricetown and Lowellville, respectively. The chloride content of the Mahoning River was well within acceptable levels. Sulfate from wastes disposal and acid mine drainage made up the largest quantity of dissolved-solids load in the Mahoning River. The sulfate load at 50-percent flow duration increased from 38 tons per day at Pricetown to 300 tons per day at Lowellville. At Pricetown the sulfate load ranged from about 2 to 588 tons per day, while at Lowellville, downstream from the industrialized area, the range was from 106 to 2,420 tons per day. Comparison of sulfate loads during periods of steel production with periods of steel-mill shutdown indicated that during low flow about half the sulfate load at Lowellville was derived from steel-mill wastes when the production index was 100. The alkalinity load of the Mahoning River at 50-percent flow duration increased from Pricetown (23 tons per day) to Lowellville (41 tons per day). During steel production the alkalinity of the water showed a marked decrease from Leavittsburg downstream to Lowellville. However, during steel-mill shutdowns the chemical composition of the river at Youngstown and Lowellville was similar to that at Leavittsburg. Acid mine drainag