Abstract The main geothermal reservoir of Acqui Terme-Visone hosts Na–Cl waters, which are in chemical equilibrium at 120–130°C with typical hydrothermal minerals including quartz, albite, K-feldspar, illite, chlorite (or smectite), anhydrite, calcite and an unspecified Ca-Al-silicate. In the Acqui Terme-Visone area, these geothermal waters ascend along zones of high vertical permeability and discharge at the surface almost undiluted or mixed with cold, shallow waters. To the SW of Acqui Terme, other ascending geothermal waters, either undiluted or mixed with low-salinity waters, enter relatively shallow secondary reservoirs, where they reequilibrate at 65–70°C. Both chemical and isotopic data indicate that bacterial SO 4 reduction affects all these waters, especially those discharged by the secondary reservoirs. Therefore, geothermal waters must get in contact with oil, acquiring the relatively oxidized organic substances needed by SO 4-reducing bacteria. This oil–water interaction process deserves further investigations, for potential economic implications.