Previous current efficiency models of the rate of aluminium production in Hall-Heroult cells are briefly summarized. A description and discussion of the cathode processes are given, and rate limiting steps are evaluated under the assumption of no concentration gradients in the bulk electrolyte phase during electrolysis. An electrochemical current efficiency model is derived from selected rate determining steps. The model expresses the local current efficiency in terms of the local partial current density for the aluminium deposition reaction and the local partial current density for all cathodic loss reactions. The model equations are valid both for laboratory and for commercial cells and independent of whether or not electrolyte impurities are involved in cyclic redox reactions. The necessary precautions to study the parameters affecting current efficiency both in commerical and in laboratory cells are outlined.