A series of (BMI)(3)LnCl(6) (Ln = Sm, Eu, Dy, Er, Yb) crystals was prepared from solutions of LnCl(3) dissolved in the ionic liquid, 1-butyl-3-methylimidazolium chloride (BMICl). Crystals with Ln = 5% Sm + 95% Gd and with Ln = 5% Dy + 95% Gd were also grown to assess the importance of cross-relaxation in the Sm and Dy samples. The crystals are isostructural, with monoclinic space group P2(1)/c and four formula units per unit cell. The first coordination sphere of Ln(3+) consists of six Cl- anions forming a slightly distorted octahedral LnCl(6)(3-) center. The second coordination sphere is composed of nine BMI+ cations. The emission spectra and luminescence lifetimes of both (BMI)(3)LnCl(6) crystals and LnCl(3) in BMICl solution were measured. The spectroscopic similarities suggest that crystalline (BMI)(3)LnCl(6) provides a good model of the Ln(3+) coordination environment in BMICl solution.