A mathematical model for a porous, silver-based electrode for the oxygen reduction in alkaline solutions, based on the thin film flooded agglomerate theory, was developed. These electrodes are employed in the energy-efficient chlor-alkali electrolysis with oxygen depolarized cathodes. The model parameters were determined from overpotentials at different oxygen concentrations obtained in half-cell measurements. For the description of the reaction kinetics, it was necessary to introduce two Tafel equations, which might be explained by a change of the adsorption isotherm of the intermediate species during oxygen reduction. The model allows for a successful description of the overpotentials in the region of industrially relevant current densities. The analysis of the oxygen concentration distribution in the liquid electrolyte reveals that massive diffusion limitations occur although the calculated size of the agglomerates is only in the range of a few micrometers.