A mathematical model has been developed to describe the steady-state constant-current operation of an oxygen cathode in aqueous carbonate electrolyte, as might be used as a component of a direct-methanol fuel cell. The diffusion and reaction of oxygen in PTFE-bonded Pt-catalyzed porous gas diffusion electrode agglomerates as well as Ohmic and migration effects over the catalyst layer were taken into account. The model accurately predicts the shape and the oxygen pressure dependence of cathode polarization data for 2 and 4 M K2CO3 electrolyte, using no adjustable parameters. Transport of OH- ions out of the catalyst layer is shown to be a limiting factor in the operation of these modern fuel cell cathodes, which have been generally optimized for maximum oxygen transport in the electrolyte. The model. indicates that higher catalyst layer porosities and thinner electrodes would yield better cathode performance in this electrolyte.