A theoretical study of current density and potential at the anode, membrane and cathode, of a chlor-alkali membrane cell where the electrode blades are placed vertically, is presented. A representative unit cell is modelled in primary, secondary and pseudo-tertiary current distribution models. It is shown that electrolyte and membrane resistance has the greatest effect on current distribution. Furthermore, it is shown that there is a surprisingly small influence of mass transport on current distribution, on the assumption that the diffusion layer is of constant thickness. In converse to this, it is shown that mass transport affects the anode overpotential distribution to the extent that conclusions can be made about the occurrence of side-reactions and where they occur. Finally, it is shown that it is possible to estimate tertiary behaviour with a secondary current distribution model, by using an analytic expression at the anode surface.