The kinetics of a range of ferrocene derivatives with horseradish peroxidase (HRP), cytochrome c peroxidase (CCP) and 3 charge reversal mutants of cytochrome c peroxidase were measured using cyclic voltammetry. Substantial differences in rate constant (100 fold) were observed between HRP and CCP for the same mediator with smaller differences (4-5 fold) for different mediators with the same enzyme. The rate constant did not seem to be dependent on redox potential differences. Cluster analysis is proposed as a way of classifying mediator reactivity.