The electrochemical reduction of Al2O3 has been investigated in molten CaCl2 at 1123 K. To predict the electrochemical reduction behavior depending on the activity of O-2-ions, the potential-pO(2)-diagram for the Al-Ca-O-Cl system is constructed from thermochemical data. In a Mo box-typeelectrode, an Al2O3 tube is successfully reduced to liquid Al with a maximum purity of 98 at%. However, in the electrolysis of Al2O3 powder in an Fe box-type electrode, Al2Ca is produced through the formation of Ca12Al14O33 as an intermediate product. The different electrochemical reduction behaviors of the tube and the powder are explained by the different diffusion path lengths for O-2-ions from three-phase zone (Al2O3/CaCl2/cathode metal) to bulk CaCl2. (c) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License.