Stable lipid bilayer membranes containing cytochrome c oxidase from two sources (i.e. bovine and Bacillus stearothermophilus) have been immobilized on gold quartz crystal microbalance (QCM) electrodes. The immobilized oxidase can be characterized using direct electrochemical methods. Amperometric flow injection analysis data of reduced cytochrome c reacting at oxidase-modified electrodes indicate that the kinetics are non-hyperbolic, which agrees with previous results found in the literature. The kinetics of the cytochrome c oxidase/cytochrome c redox couple are characterized by a high and low affinity regime each having a distinct K-m value. The rate of mediated electron transfer from solution-resident reduced cytochrome c to the electrode is also affected by ionic strength. Electrochemical and QCM results suggest that the fastest rates of electron transfer are observed when the ionic strength is low enough to allow facile binding between the protein and the enzyme but high enough to allow the protein to dissociate from the oxidase.