The U.S. Geological Survey (USGS) in cooperation with the Miami Conservancy District, Dayton, Ohio, investigated concentrations of per- and polyfluoroalkyl substances (PFAS) in groundwater from the Great Miami buried-valley aquifer (GM-BVA) of southwestern Ohio in 2019 and 2020. Data in this release include redox (reduction/oxidation) related water-chemistry determinations from groundwater sampled previously in 1999 and 2000 and from groundwater sampled by this study in 2019 and 2020 that include concentrations of dissolved oxygen, sulfate, several nitrogen species, manganese, and iron. The 23 wells used for the sampling were identified and sampled previously by the USGS National Water-Quality Assessment Program to assess concentrations of a variety of inorganic and organic contaminants in the GM-BVA. Sampled wells and prior data were from parts of the GM-BVA in Butler, Champaign, Clark, Greene, Hamilton, Miami, Montgomery, Shelby and Warren counties in the Great Miami River, Little Miami River, and Whitewater River Basins. Classifications of redox category and redox process derived from a procedure implemented by Jurgens and others (2009) and applied to these water chemistry determinations are included in the data release. The redox related water-chemistry determinations as originally derived are also provided with the Excel® workbook published by Jurgens and others (2009). Data in the attached Excel® workbook with dissolved oxygen concentrations that were less than the practical limit of quantitation or concentrations of nitrate plus nitrite, ammonia, manganese or iron that were less than the reporting limit for an analysis were set equal to the practical limit of quantitation or the reporting limit. A redox process was not identified for one analysis that lacked an iron analytical result.