Mississippi Valley-type (MVT) sulfide minerals and oil from deposits along the Cincinnati arch have almost identical δ34S values (-9‰ to +9‰ for MVT sulfides; -12‰ to +9‰ for oils). These values are very similar to those for MVT sulfides and oil in the Illinois-Kentucky district and support their proposed inclusion in a regional hydrothermal system. Many MVT deposits with low δ34 values are closely associated with oil, whereas MVT deposits with high δ;34 values often contain bitumen. Reduced sulfur in MVT deposits with high δ34 values probably came from thermochemical sulfate reduction, whereas that in MVT deposits with low δ34 values probably came from oil and related organic matter. Oil-related sulfur could have been derived from oil fields or disseminated oil and other organic matter in regional wallrocks.