The source of the background current in the usual operation of the electrochemical concentration cell ozonesode is the reduction of tri-iodide (iodine) normally present in the cathode solution. We can explain the time variations in the background current by the slow rates of solution mass transport and of heterogeneous electron transfer for tri-iodide. Oxygen does not contribute to the background current through reaction with iodide to produce tri-iodide. Direct reduction of oxygen at the cathode is negligible once the electrodes have been exposed to iodide for 24 hours. The present background current correction is altitude-dependent, since it is based on an assumed sensitivity of the electrochemical concentration cell to oxygen. If the background current is independent of altitude and of oxygen, a constant, uncorrected, background current of 0.1 μA would produce an error of +5.5% for the total ozone value of the electrochemical concentration cell for a typical mid-latitude total ozone value of 0.345 atm cm.