Microbial fuel cell (MFC) air cathodes examined here were made using poly(phenylsulfone) (Radel (R)) binders sulfonated to various ion exchange capacities (IECs). We examined the effect of increasing the IEC of poly(phenylsulfone) Radel binders from 0 to 2.54 meq/g on cathode performance using linear sweep voltammetry (LSV), impedance, and single chamber air-cathode MFC tests. Unsulfonated Radel, which is a non-ionic, hydrophobic polymer, showed the highest current in LSV tests and the lowest charge transfer resistance. Increasing the binder IEC resulted in a decreased current response in LSV tests and an increased charge transfer resistance from 8 to 23 Omega. It is proposed that the presence of sulfonate groups in the cathode binder impeded the oxygen reduction activity of the cathodes by adsorption of the sulfonate to catalytic sites and by impeding proton diffusion to the catalyst surface. The unsulfonated Radel binder produced the most stable performance, and eventually the highest power density, in MFCs operated over 20 cycles (55 days). These results suggest that the use of a non-ionic binder is advantageous in an MFC cathode to facilitate charge transfer and stable performance in the neutral pH conditions found in MFCs. (C) 2010 Elsevier Ltd. All rights reserved.