The effect of relative water content on the luminescence properties and speciation of Eu3+ ions in solutions of EUCl3 in the binary solvent mixture water/[BMI]Cl is presented, where [BMI]Cl is the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride. Using luminescence techniques, the binding properties of water to Eu3+ are determined for samples with mole ratios of water-to-IL ranging from 0 to 5. Very little water binds to Eu3+ at mole ratios of water-to-IL less than 1, above which binding increases rapidly with increasing water concentration. It is shown that only certain hydration numbers for Eu3+ complexes are stable in the water/IL solutions. The data presented suggest that the Eu3+ species present are [EuClx](3-x), [EuCly(H2O)(3-4)](3-y) [EuCl2(H2O)(6)](3-z), and [Eu(H2O)(8-9)](3+) (where x > y > z). Comparison of the positions of the D-5(0)<- F-7(0) transitions of the Eu3+ complexes in IL solution with those of model crystal systems provides insight into the extent of Cl- complexation. This study suggests that [BMI]Cl is a promising medium for luminescent lanthanide (Ln) compounds due to the low-energy phonon environment of the [LnCl(x)](3-x) complex and to the fact that moderate water contamination does not result in direct binding of water to Ln(3+), which would result in luminescence quenching.