An experimental investigation was carried out on the kinetic and stability behaviour of Aspergillus niger glucose oxidase in water-polyol mixtures. Kinetic runs performed in systems containing up to 8 M polyhydric cosolvents (ethylene glycol, propylene glycol and glycerol) showed a strong dependence of the catalytic properties of the enzyme on the nature and composition of the reaction medium. Higher residual activities were measured in the presence of additives of lower polarity, according to the following order : propylene glycol > ethylene glycol > glycerol. Dilution and thermal unfolding experiments provided evidence that the activity reductions were not ascribable to solvent-induced structural modifications of the enzyme molecule but, rather, to the inhibitory action exerted by the polyols.