The cause of the incipience of the anode effect in alumina reduction cells has long been a controversial issue. The most plausible interpretations are (i) an insufficient release of the gaseous phase from underneath the anode surface together with the action of diminished wettability and (ii) the depletion of oxygen-containing ions at the anode surface to result in the limiting current condition. It is shown by means of a mathematical model that each of these effects may be active. The predominant mechanism depends on the operational conditions. Theoretical results are compared with experimental data.