A numerical simulation of an enzyme-catalyzed oxygen cathode is presented and applied to the analysis of transport limitations in operating electrodes, with the goal of predicting the limits of obtainable cathode current density. Based on macrohomogeneous and thin-film theories, and accounting for dual-substrate enzyme kinetics, the one-dimensional model predicts a maximum current density of about 9.2mAcm−2 at 0.6V (SHE) for a 300μm thick electrode operating oxygen-saturated pH 5 buffer at 37°C and relying on diffusion of dissolved oxygen alone. However, by introducing gas-phase diffusive transport, or alternatively a convective, flow-through approach, the model predicts that electrodes of identical thickness may provide current densities up to 60mAcm−2 in air and exceeding 100mAcm−2 in pure O2. Such performance would move enzyme electrodes closer to practical implementation in implantable power devices and other low-temperature fuel cells such as direct methanol fuel cells.