Catalysis of the electrochemical oxidation of glucose by glucose oxidase with a single electron mediator (cosubstrate) may be used to transform mixtures of concentrated industrial sugars. How the high viscosity of such media may affect the enzymatic reaction and the transport of the mediator can be mimicked by addition of large concentrations of sucrose to glucose solutions, Cyclic voltammetry then provides a simple means of investigating the effect of an increased viscosity on the kinetics of the enzymatic reaction and the diffusion of the mediator. The diffusion coefficient of the mediator is decreased 10 times by addition of 1.6 M sucrose, At pH 8, in the presence of the same concentration of sucrose, the catalytic activity of the enzyme towards its substrate is only slightly affected, A 35% decrease of the glucose Michaelis constant is observed, The reaction of the reduced enzyme with the cosubstrate is six times slower and the mediator Michaelis constant undergoes a three-fold increase. It follows that glucose oxidase remains an efficient catalyst in such viscous media.