The study investigates the role of flavins, specifically riboflavin and riboflavin-5′-phosphate, in extracellular electron transfer (EET) by *Shewanella* species. Using electrochemical techniques, the researchers found that these flavins are actively secreted by *Shewanella oneidensis* MR-1 and *Shewanella sp.* MR-4 biofilms and mediate electron transfer to electrodes. The removal of riboflavin from the biofilms significantly reduced the electron transfer rate by over 70%, indicating its crucial role as a soluble redox shuttle. Differential pulse voltammetry and cyclic voltammetry revealed that flavins adsorb to electrode surfaces, even after soluble components are removed, particularly in older biofilms. Riboflavin also binds to geochemically relevant surfaces like Fe(III) and Mn(IV) oxides. The findings suggest that the production and binding of flavins by *Shewanella* are essential for their ability to transfer electrons to metals, and that these compounds may play a more complex role in extracellular respiration than previously thought.The study investigates the role of flavins, specifically riboflavin and riboflavin-5′-phosphate, in extracellular electron transfer (EET) by *Shewanella* species. Using electrochemical techniques, the researchers found that these flavins are actively secreted by *Shewanella oneidensis* MR-1 and *Shewanella sp.* MR-4 biofilms and mediate electron transfer to electrodes. The removal of riboflavin from the biofilms significantly reduced the electron transfer rate by over 70%, indicating its crucial role as a soluble redox shuttle. Differential pulse voltammetry and cyclic voltammetry revealed that flavins adsorb to electrode surfaces, even after soluble components are removed, particularly in older biofilms. Riboflavin also binds to geochemically relevant surfaces like Fe(III) and Mn(IV) oxides. The findings suggest that the production and binding of flavins by *Shewanella* are essential for their ability to transfer electrons to metals, and that these compounds may play a more complex role in extracellular respiration than previously thought.