The preparation of nickel powders by electro-deoxidation of nickel oxide in molten sodium hydroxide at 550 degrees C has been investigated. Cyclic voltammetry on different electrodes was applied to study the electrochemical reaction mechanism of the reduction of NiO in the melt. In addition, NiO pellets were reduced by constant voltage electrolysis for different times. The results disclose that the reduction of nickel oxide is a one-step reaction and the side reaction, hydrogen evolution, may lower the current efficiency during the constant voltage electrolysis. The current efficiency increases from 25.9% to 87.2% with the addition of sodium oxide which leads to a higher cathodic potential for the hydrogen evolution reaction and, therefore, prevents it occurring. With using an inert nickel anode, oxygen gas emits during the electrolysis. (C) The Author(s) 2015. Published by ECS. All rights reserved.