Black crystals of the bismuth subchloride Bi₇Cl₁₀ were obtained by slow cooling of stoichiometric mixtures of Bi und BiCl ₃ from 500°C to ambient temperature. By means of thermal analyses the phase diagram Bi/BiCl₃ was specified. Bi₇Cl ₁₀ decomposes peritectoidally into Bi₆Cl₇ and BiCl₃ at 190 ± 5°C. The phase barogram of the binary system was determined by total pressure measurements in the membrane zero-manometer. From the pressure functions of the bismuth chlorides as well as from measurements of the molar heat of Bi₆Cl₇ the thermodynamic standard data were derived: ΔH°_B(Bi ₆Cl₇, 298) = -895 ± 5 kJ·mol^-1; S°(Bi₆Cl₇, 298) = 620 ± 10 J·mol ^-1 K^-1; ΔH°_B(Bi₇Cl ₁₀, 298) = -1310 ± 10 kJ·mol^-1; S°(Bi₇Cl₁₀, 298) = 730 ± 20 J·mol ^-1 K^-1. Using these data, thermodynamic modelling of the solid-gas-equilibria were carried out in order to optimize the synthesis of Bi₇Cl₁₀. The phase-pure vapour deposition of Bi ₇Cl₁₀ is impossible due to the condensation of the dominating gas-species in the stability range of the compound. X-ray diffraction on single-crystals at room temperature revealed that Bi₇Cl ₁₀ crystallizes with the tetragonal space group I 4₁/a c d and the lattice parameters a = 28.235(3) and c = 39.950(4) Å (Z = 64). In the crystal structure, polycations Bi₉⁵⁺ with the shape of tri-capped trigonal prisms are embedded in a chloro-bismuthate(III) framework _∞³[Bi₅Cl₂₀^5-].