An isothermal, one-dimensional transient model of a porous oxygen electrode in an alkaline medium was developed, based on mass balances for oxygen and ionic species and flooded-agglomerate theory. The model is in reasonable agreement with experimental data. including an observed potential-dependent time constant. Thus, for small cathodic steps from the open-circuit potential, time constants on the order of 1 s may be expected. For larger cathodic steps (i.e., to lower potentials), the time constant will be substantially smaller, on the order of milliseconds. The model also shows that there is room for dynamic improvement of the cathodes without sacrifice of a steady-state response. An overshoot in the faradaic current seen for the majority of the calculations is explained as due to the combined effects of fast potential propagation and slow oxygen diffusion. (C) 2009 The Electrochemical Society.