Glucose oxidase and ferro/ferricyanide can be co-immobilised in poly(N-methylpyrrole) films. The two species compete for incorporation into the films so that on increasing the concentration of enzyme in the growth solution less ferro/ferricyanide is incorporated into the film, and vice versa. The ferro/ferricyanide entrapped within the film acts as a redox mediator for the oxidation of the enzyme at 0.45 V vs. SCE. Studies of the effect of variation of film thickness, glucose concentration, enzyme loading and ferro/ferricyanide loading are consistent with a model in which diffusion and enzyme-catalysed reaction within the film are coupled. Analysis of data from these experiments allows the rate parameters for these processes to be determined. These data are compared with previous results for oxygen mediation in poly(N-methylpyrrole) films.