The activity coefficients of rare earth chlorides in molten salt systems are of great interest for the research, development and commercialization of pyrochemical processing of spent nuclear fuels. In this paper, the activity coefficient of GdCl3 in molten LiCl-KCl eutectic salt has been studied using a two electrode electrochemical cell with a gadolinium metal rod as working electrode and a saturated Gd|GdCl3-LiCl-KCl reference electrode. Use of this analyte reference electrode allows for the elimination of hereto unavoidable errors in the experiments. It is determined that the activity coefficient of GdCl3 is a function of concentration; decreasing from 3.9 x 10(-4) at low concentrations to 4.9 x 10(-5) at about 0.62 mol % GdCl3, remaining unchanged up to about 2 mol % GdCl3 after which it begins to increase with increasing concentration of GdCl3. Further it is also determined experimentally that presence of high levels of CsCl tends to decrease the activity coefficient of GdCl3. (C) The Author(s) 2017. Published by ECS. All rights reserved.