The use of redox potential measurements for corrosion and scaling monitoring, including microbially mediated processes, is demonstrated. As a case study, monitoring data from 10. years of operation of an aquifer thermal energy storage (ATES) site located in Berlin, Germany, were examined.(Fe ²⁺)-activities as well as [Fe ³⁺]-build up rates were calculated from redox potential, pH, conductivity, temperature and dissolved oxygen measurements. Calculations are based on assuming (Fe ³⁺)-activity being controlled by Fe(OH) ₃-solubility, the primary iron(III)-precipitate. This approach was tested using a simple log-linear model including dissolved oxygen besides major Fe ²⁺-ligands. Measured redox potential values in groundwater used for thermal storage are met within ±8mV. In other systems comprising natural groundwater and in heating and cooling systems in buildings, quantitatively interpretable values are obtained also.It was possible to calculate particulate [Fe ³⁺]-loads in the storage fluids in the order of 2μM and correlate a decrease in filter lifetimes to [Fe ³⁺]-build up rates, although observations show clear signs of microbially mediated scaling processes involving iron and sulphur cycling.