Bacteria reducing extracellular acceptors utilize immobilized mediators or soluble shuttles. Investigation of two soluble compounds, anthraquinone-2,6-disulfonate and riboflavin, revealed similar redox potentials and diffusion coefficients. However, riboflavin showed more facile interfacial kinetics. Additions of each shuttle caused dose-dependent increases in current production by Shewanella oneidensis. The two shuttles behaved similarly at high applied potentials, but riboflavin mediated higher current at lower applied potentials. These results demonstrate how electron shuttle behavior can be diffusion-controlled at higher potentials, while interfacial kinetics can become dominant at potentials closer to these compounds' midpoint potentials. In general, the fact that added shuttles increased electron flux as much as 10-fold shows substrate oxidation is not always the rate-limiting step in electron transfer to electrodes. The potential-dependent behavior highlights differences that likely manifest during both environmental mineral reduction and in microbial fuel cells operated to achieve maximum power density.