Quaternization of poly-N-vinylimidazole partly complexed with [Os(4,4(')-dichloro-2,2(')-bipyridine)(2)Cl](+/2+) produced, after cross-linking with polyethylene glycol diglycidyl ether, a redox hydrogel with a redox potential of 0.38 V vs Ag/AgCl and an apparent electron diffusion coefficient of 1.5x10(-7) cm(2) s(-1) at 37.5 degrees C. Electrostatic binding and cross-linking with bilirubin oxidase from Trachyderma tsunodae provided a mechanically robust O-2-electroreduction catalyst withstanding a shear stress of 2x10(-2) N m(-2) associated with the rotation of a 3.6 mm radius electrode at 500 rpm. Unlike other catalysts made with osmium complexes, comprising in their inner coordination sphere imidazole-exchangeable chloride, it did not appreciably deteriorate by cross-linking through ligand exchange, leading to dehydration. The threshold potential of O-2-electroreduction at pH 7.4, 0.15 M NaCl, and 37.5 degrees C was near that calculated for the reversible O-2/H2O half-cell. The O-2-electroreduction current reached half of its maximum at +0.44 V vs Ag/AgCl, 60 mV more positive than the potential for bilirubin oxidase wired with PAA-PVI-[Os(diCl-bpy)(2)Cl](+/2+) (PAA=polyacrylamide, PVI=poly-N-vinylimidazole).