Quaternary ammonium bromide salt-treated Nation membranes provide an ideal environment for enzyme immobilization. Because these quaternary ammonium bromide salt-treated Nafion membranes retain the physical properties of Nation and increase the mass transport of ions and neutral species through the membrane, they are also ideal for modifying electrodes. Therefore, high current density bioanodes are formed from poly(methylene green) (an electrocatalyst for NADH) modified electrodes that have been coated with a layer of tetrabutylammonium bromide salt-treated Nafion with dehydrogenase enzymes immobilized within the layer. Ethanol/O-2 biofuel cells employing these bioanodes have yielded power densities of 1.16 mW/cm(2) with a single-enzyme system (alcohol dehydrogenase) and 2.04 mW/cm(2) with a double-enzyme system (alcohol dehydrogenase and aldehyde dehydrogenase) in the polymer layer. Methanol/O-2 biofuel cells employing these bioanodes have yielded power densities of 1.55 mW/cm(2) and open circuit potentials of 0.71 V. (c) 2004 Elsevier Ltd. All rights reserved.