A mathematical model for a chlor-alkali electrolysis cell with oxygen-depolarized cathode (ODC) was developed. With this model, distributions of temperature, concentration, current density, and overpotential as a function of height can be calculated. At an industrially relevant current density of , neither current density nor overpotentials exhibit strong variations along the cell height. Main reason for this behavior is the uniformity of temperature distributions in the solid compartments of the cell (anode, membrane, ODC) which can be explained by efficient heat transfer between the electrodes and the electrolyte streams. This is especially true for the caustic solution, through which most of the irreversible heat released in the cell is removed. However, the temperature of the oxygen stream increases only slowly until at the top of the electrolyzer, gas temperatures around 78 A degrees C are achieved. Due to the initially low temperatures and the low water content of the inlet oxygen stream, the gas phase takes up considerable net amounts of water vapor. Nevertheless, the oxygen partial pressure at the electrochemically active regions of the ODC remains high allowing for efficient operation of the cathode.