The viability of Lewis-acid ionic liquids for the synthesis of low-valent bismuth compounds is demonstrated. At room temperature, elemental bismuth and bismuth(III) cations synproportionate in the ionic liquid [BMIM]Cl/AlCl ₃ ([BMIM]⁺: 1-n-butyl-3-methylimidazolium) within minutes. The existence of bismuth polycations in the dark colored solution was proven by Raman spectroscopy. Dark-red crystals of Bi₅(AlCl₄) ₃ were isolated from the ionic liquid and characterized by Raman spectroscopy and X-ray crystallography (rhombohedral space-group R3c, a = 1187.1(2) pm, c = 3012.0(3) pm). The method allows the synthesis of bismuth cluster compounds under milder conditions than in high-temperature melts and more conveniently and environmental friendly than in liquid SO₂ with strongly oxidizing, toxic agents like SbF₅ or AsF₅.