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Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recovery of lanthanides from primary ores and electronic waste. However, they require the speciation of dissolved elements to be known with accuracy. In the present study, Eu coordination and Eu III /Eu II electrochemical behavior as a function of water content in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf 2 ]) was investigated using UV-visible spectrophotometry, time-resolved laser fluorescence spectroscopy, electrochemistry, and X-ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemical cells. Under anhydrous conditions, Eu III and Eu II were complexed by NTf 2 , forming Eu-O and Eu-(N,O) bonds with the anion sulfoxide function and N atoms, respectively. This complexation resulted in a greater stability of Eu II , and in quasi-reversible oxidation-reduction with an E 0 ' potential of 0.18 V versus the ferrocenium/ferrocene (Fc + /Fc) couple. Upon increasing water content, progressive incorporation of water in the Eu III coordination sphere occurred. This led to reversible oxidation-reduction reactions, but also to a decrease in stability of the +II oxidation state (E 0 '=-0.45 V vs. Fc + /Fc in RTIL containing 1300 mm water)., (© 2020 Wiley-VCH GmbH.)