The transformation of Eu(III) to Eu(II) was confirmed in a fluoride eutectic, LiF-NaF-KF (46.5-11.5-42.0 mol%, FLiNaK) molten salt during a treatment of high temperature as high as 1023 K. The coexistence of Eu(III)-Eu(II) was characterized by X-ray photoelectron spectroscopy (XPS) and voltammetry method, and their concentrations were measured. The electrochemical behavior of Eu(III) and Eu(II) in the fluoride salt was investigated. The mechanism of the electrode reaction was determined using cyclic voltammetry (CV) and square wave voltammetry ( SWV). The results indicated a one-electron exchange process, corresponding to the reduction of Eu(III) to Eu(II) and the oxidation of Eu(II) to Eu(III). This process is reversible and diffusion-controlled. The diffusion coefficients (D) of Eu(III) and Eu(II) were determined using the conventional CV by changing the scanning rate and a modified method by changing the area of the working electrode successively. The values obtained by these two different methods were consistent. The temperature dependence of diffusion coefficient was investigated, and the activation energies of diffusion process were calculated to be 38.9 + 4.6 kJ mol(-1) for Eu(III) and 34.7 + 1.6 kJ mol(-1) for Eu(II), respectively. (C) 2014 Elsevier Ltd. All rights reserved.