Anode-respiring bacteria (ARB) in a biofilm anode carry out an oxidation half-reaction of organic matter, producing an electrical current from renewable biomass, including wastes. At the same time, ARB produce protons, usually one proton for every electron. Our study shows how current density generated by an acclimated ARB biofilm was limited by proton transport out of the biofilm. We determined that, at high current densities, protons were mainly transported out of the biofilm by protonating the conjugate base of the buffer system; the maximum current generation was directly related to the transport of the buffer, mainly by diffusion, into and out of the biofilm. With non-limiting acetate concentrations, the current density increased with higher buffer concentrations, going from 2.21 ± 0.02 A m−2 with 12.5-mM phosphate buffer medium to 9.3 ± 0.4 A m−2 using a 100-mM phosphate buffer at a constant anode potential of Eanode = −0.35 V versus Ag/AgCl. Increasing the concentration of sodium chloride in the medium (0–100 mM) increased current density by only 15%, indicating that ion migration was not as important as diffusion of phosphate inside the biofilm. The current density also varied strongly with medium pH as a result of the buffer speciation: The current density was 10.0 ± 0.8 A m−2 at pH 8, and the pH giving one-half the maximum rate was 6.5. A j–V curve analysis using 100 mM phosphate buffer showed a maximum current density of 11.5 ± 0.9 A m−2 and half-saturation potential of −0.414 V versus Ag/AgCl, a value that deviated only slightly from the standard acetate potential, resulting in small anode-potential losses. We discuss the implications of the proton-transport limitation in the field of microbial fuel cells and microbial electrolytic cells. Biotechnol. Bioeng. 2008;100: 872–881. © 2008 Wiley Periodicals, Inc.