Electrodes modified with co-immobilized redox enzymes and redox polymers can be used to form membrane-less biofuel cells. In this communication, we report on our initial studies of a membrane-less biofuel cell concept using an osmium-based redox polymer for laccase-mediated reduction of oxygen coupled to glucose oxidase-mediated oxidation of glucose. We then present a thermodynamic examination of mediators of laccase oxygen reduction, and stemming from this, target two redox polymers of potential use, an osmium-based redox polymer (E-0' + 0.40 V vs. Ag/AgCl) and a ruthenium-based redox polymer (E-0' + 0.63 V vs. Ag/AgCl). The former shows promise for use in membrane-less biofuel cell cathodes, whilst the latter's redox potential is too high to be an effective mediator of oxygen reduction by the Trametes versicolor laccase used in this study. (C) 2003 Elsevier B.V. All rights reserved.