Oxygen was electrocatalytically reduced to water at a current density of 5 mA/cm(2) and at +0.7 V (NHE) in pH 5 citrate buffer at 37.5 degreesC. The electrocatalyst was a composite of laccase cross-linked with a redox. polymer on a hydrophilic cloth of 10 Aim diameter carbon fibers. The redox polymer, PVI-Os(tpy)(dme-bpy)(2+/3+), [poly-N-vinyl imidazole with 1/5(th) of the imidazoles complexed with [Os(tpy)(dme-bpy)]2+/3+ (tpy=terpyridine; dme-bpy= 4,4'-dimethyl-2,2'-bipyridine)], electrically connected ("wired") the laccase reaction centers to the fibers.