The Gulf Coast Regional Aquifer-System Analysis is a study of regional aquifers composed of sediments of mostly Cenozoic age that underlie about 230,000 sq mi of the Gulf Coastal Plain. These regional aquifers are part of three aquifer systems: (1) the Mississippi Embayment Aquifer System, (2) the Texas Coastal Uplands Aquifer System, and (3) the Coastal Lowlands Aquifer System. The water chemistry of the Middle Wilcox Aquifer, which is part of the Mississippi Embayment Aquifer System and the Texas Coastal Uplands Aquifer System is presented by a series of maps. These maps show the area1 distribution of (1) the concentration of dissolved solids and temperature, (2) the primary water types and pH, (3) the concentration of major ions and silica, and (4) the milliequivalent ratios of selected ions. Dissolved constituents, pH, temperature, and ratios are based on the median values of all samples in each 100-sq-mi area. The concentration of dissolved solids in water from the Middle Wilcox Aquifer ranges from 26 mg/L in the northern part of the Mississippi Embayment Aquifer System to 125,500 mg/L in a down-dip area in southeastern Texas. The primary water types, which are based on the most frequently observed type in each 100-sq-mi area, are calcium bicarbonate in the outcrop in Missouri, Kentucky, Mississippi, and southern Texas; sodium bicarbonate in the remaining outcrop areas and all areas from outcrop to mid-dip; and sodium chloride in all down-dip areas. The concentrations of major ions in water from the Middle Wilcox Aquifer generally increase from the outcrop area to the down-dip limit of the data. The milliequivalent ratio of magnesium plus calcium to bicarbonate ranges from less than 0.01 to 158 and generally decreases from outcrop to mid-dip and increases from mid-dip to the down-dip limit of the data. From the Sabine Uplift eastward to southwestern Alabama the ratio of bicarbonate to chloride generally decreases from outcrop to down-dip in the area west of the Mississippi River and shows localized trends east of the Mississippi River.