Reduction-oxidation (redox) reactions are central to environmental chemical processes. Many redox reactions occur as sequences (e.g., photosystem II, which provide electrons for photosynthesis), with the intermediate species referred to as electron transfer mediators (ETMs). ETMs can be any molecule that helps transfer electrons from one chemical moiety to another. In the laboratory, ETMs are more specific, such as species that are well characterized, and undergo fast electron transfer in both oxidation and reduction reactions (reversible). Here, various electrochemical methods were used to characterize environmentally relevant ETMs, such as phenols and anilines, natural organic matter, and various iron oxides.