Iron-bearing clay minerals are ubiquitous in the environment and have been shown to play important roles in several biogeochemical processes. Previous efforts to characterize the Fe2+–Fe3+ redox couple in clay minerals using electrochemical techniques have been limited by experimental difficulties due to inadequate reactivity between clay minerals and electrodes. The current work overcomes this limitation by utilizing organic electron transfer mediators that rapidly transfer electrons with both the Fe-bearing clay minerals and electrodes. Here, an Fe-rich source clay mineral (ferruginous smectite, SWa-1) is examined with respect to what fraction of structural Fe participates in oxidation/reduction reactions and the relationship between bulk Fe2+/Fe3+ ratios to the reduction potential (Eh).