Wide applications of anhydrous rare-earth (RE) trichlorides RECl₃ in organometallic chemistry, for the synthesis of optical and magnetic materials, and as catalysts require a facile approach for their synthesis. The known methods use or produce toxic substances, are complicated and have limited reliability and upscaling. It has been shown that task-specific ionic liquids (ILs) can dissolve many metal oxides without special reaction conditions at moderate temperature, making the metals accessible to downstream chemistry. Using imidazolium chloridoaluminate ILs, pure crystalline anhydrous RECl₃ (RE=La−Nd, Sm−Dy) can be synthesized in one step from RE oxides in high yield. The Lewis acidic IL acts as solvent and reaction partner. The by-product [Al₄O₂Cl₁₀]²⁻, which was detected spectroscopically, remains in solution. The reacted IL can be removed quantitatively by washing. ILs with various imidazolium cations and AlCl₃ content and the effect of temperature and reaction time were tested.