Iron-sulfur geochemistry is important in many natural and engineered environments, including drinking water systems. In the anaerobic environment beneath scales of corroding iron drinking water distribution system pipes, sulfate reducing bacteria (SRB) produce sulfide from naturally-occurring sulfate in the bulk water. The effect of bacterial sulfate reduction on the nature of iron corrosion scales collected from a groundwater drinking water distribution system having an aqueous sulfate concentration of about 100 mg/L was studied. Reduced sulfur proved to be a significant component of all iron scales studied, reaching up to 22% (wt. %) in the scale. X-ray diffraction revealed the presence of elemental sulfur and FeS₂ (marcasite). Other important iron minerals found were goethite ( -FeOOH) and lepidocrocite ( -FeOOH). Magnetite (Fe₃O₄), which is believed to be an important mineral in protecting iron pipes from extensive corrosion and iron release, was not present in the iron scales. The dominant reaction pathway operating in these scales was believed to be oxidation of HS- by FeOOH to S^o, followed by reaction of S^o with FeS to make FeS₂, consuming all of the FeS in the process and leaving excess S^o. Includes 15 references, tables, figures.