New lanthanide-containing ionic liquids (ILs) based on thiocyanate complexes of the type [BMIM]x–3[Ln(NCS)x(H2O)8–x] (BMIM = 1-butyl-3-methylimidazolium; x = 6,7,8) have been reported in recent studies. These complexes are highly efficient for dissolving lanthanide complexes within ILs. Until now, only partial characterization of their properties has been reported. The present work describes the characterization of this type of complexes in both their pure liquid phase and in solution with EuIII and SmIII as the lanthanide ions. The number of water molecules as ligands was determined by luminescence measurements in the UV/Vis region. The composition of the complexes in solution was found to be similar to those in the solid state. The electrochemical behavior of the EuIII/EuII redox couple for [BMIM]5[Eu(NCS)8] at a glassy carbon electrode has been investigated in acetonitrile and in the [BMIM][SCN] IL. This couple is electrochemically quasireversible with E1/2 = –0.23 V [vs. normal hydrogen electrode (NHE)]. The diffusion coefficients of the complexes in solution were determined by cyclic voltammetry and chronopotentiometry and were found to be D = 1.1 ± 0.1 × 10–5 and 2.5 ± 0.3 × 10–8 cm2/s in acetonitrile and [BMIM][SCN], respectively. The diffusion coefficients of the reduced species were found to be D = 8.2 ± 0.4 × 10–6 and 2.1 ± 0.4 × 10–8 cm2/s in the two solvents, respectively. The standard rate constants of the single electron reduction were determined by cyclic voltammetry and found to be k0 = 1.2 ± 0.1 × 10–3 and 5.4 ± 0.8 × 10–5 cm/s in the two solvents, respectively.