The novel Bi-containing reactive ionic liquid [C₄mim]₃[Bi₃I₁₂], which was synthesized in quantitative yield by equimolar reaction of BiI₃ and [C₄mim]I, was used as a novel Bi-source for the ionothermal synthesis of Bi₂Te₃ nanoparticles by reaction with (Et₃Si)₂Te in the ionic liquid [C₄mim]I. The solid state structure of [C₄mim]₃[Bi₃I₁₂] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis of the single source precursor (Et₂Sb)₂Te and [C₄mim]₃[Bi₃I₁₂] yielded the ternary (Bi_xSb_1−x)₂Te₃ (x = 0.25, 0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials were determined by EDX and XRD and the surface composition of the nanoparticles was further investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM and TEM.