In this report, it is shown that novel thin films of Os(dcbpy)(2) (dcbpy = 4,4'-dicarboxylic acid-2,2'-dipyridine)-based redox polymer-laccase composite can be electrodeposited onto carbon electrodes under mild conditions. In a nutshell, the exchange of the inner-sphere Cl- of the Os(dcbpy)(2)Cl+/2+ complex tethered to partially quaternized poly (4-vinylpyridine) (PVP) by a pyridine ligand of a second PVP chain leads to cross-linking and deposition of the redox polymer. Laccase, which has coordinatively linkable functions of amines and histidines, is readily incorporated in the electrodeposited redox polymer. Because the reaction centers of the co-deposited laccase are electrically connected to the electrode through the deposited redox polymer, the electrodeposited film can catalyze the electroreduction of O-2 at 0.58 V (vs. Ag/AgCl) - the least reducing potential for highly efficient four-electron reduction of O-2 in pH 5.5 0.10 M phosphate buffer solution. Furthermore, the electroreduction of O-2 is found to be O-2 transport-limited when the reduction potential is poised at >= 120 mV more reducing than that of the reversible O-2/H2O couple. This composite film could be an excellent candidate for uses as cathode in enzymatic biofuel cells. (C) 2012 Elsevier B.V. All rights reserved.