This paper describes the influence of ZrCl4, which is electrochemically inert under experimental conditions, on uranium dioxide current efficiency during the (NaCl-KCl)(equim)-UO2Cl2 (10-20wt.%)-ZrCl4 (0-12 wt.%) melt electrolysis at temperatures of 700 and 750 C and at electrolysis current densities of 0.08-0.63 A/cm(2). An extreme dependence of the uranium dioxide current efficiency on the ZrCl4 concentration and the corresponding minimum and maximum was found. The ZrCl4 influence on the UO2 current efficiency is assumed to be caused by the changes in the mechanism of UO22+ ion reduction to uranium dioxide. (C) 2014 Elsevier Ltd. All rights reserved.