- Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars) of both natural and industrial origin. Additions of chars into soils are discussed as a strategy to mitigate climate change. Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and enviro-technical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700°C) show the highest capacities to accept and donate electrons. The electron accepting capacities (EACs) increase with the nominal carbon oxidation state of the chars. Comparable trends of EACs and of quinoid C=O contents with HTT suggest quinoid moieties as major electron acceptors in the chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".