The multi-heme cytochromes from Shewanella oneidensis associated with the dissimilatory metal reduction (DMR) pathway have been investigated using the technique of protein film voltammetry (PFV). Using PFV, we have interrogated each of the multi-heme cytochromes (MtrA, STC, and solubilized versions of the membrane-bound proteins CymA, OmcA, and MtrC) under identical conditions for the first time. Each cytochrome reveals a broad envelope of voltammetric response, indicative of multiple redox cofactors that span a range of potential of approximately 300 mV. Our studies show that, when considered as an aggregate pathway, the multiple hemes of the DMR cytochromes provide a "window" of operating potential for electron transfer to occur from the cellular interior to the exterior spanning values of -250 to 0 mV (at circumneutral values of pH). Similarly, each cytochrome supports interfacial electron transfer at rates on the order of 200 s(-1). These data are taken together to suggest a model of electron transport where a wide window of potential allows for charge transfer from the cellular interior to the exterior to support bioenergetics.