It is an old and generally accepted finding that the anode effect in alumina reduction cells is linked with the existence of a gas film underneath the anode surface. It has further been confirmed that the critical current density related to the incipience of the anode effect depends upon the wettability of the electrode. Further, hydrodynamic effects have been suggested active in the anode effect. In the present paper a mathematical model is established to find the parameters controlling the critical current density and to describe their action quantitatively. The theoretical result is compared with experimental findings confirming the outstanding effect of the contact angle, but also evidencing the additional effect of electrode geometry, mass transfer and flow conditions of the bubble layer. The number of available interpretations of the anode effect is not augmented by another one, but the result points out the combined action of wettability and hydrodynamics.