Thermal stability and phase formation of the clathrate phase BaGe5 and its formation from the clathrate-I phase Ba8Ge43□3 (□ = vacancy in the crystal structure) have been investigated by microstructure analysis, differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD). The experiments confirm the peritectic formation of the high-temperature phase Ba8Ge43□3 from α-Ge and liquid at 810 °C, and the eutectic temperature of Ba8Ge43□3 and Ba6Ge25 at 808 °C. At T = 770 °C, Ba8Ge43□3 decomposes by a eutectoid reaction to Ba6Ge25 and α-Ge, and at T = 520 °C, the low-temperature phase BaGe5 forms in a peritectoid reaction from these two phases. BaGe5 was synthesized by decomposition of the high-temperature clathrate-I phase Ba8Ge43□3 at temperatures between 350 °C and 520 °C. Annealing and DSC experiments verify that BaGe5 is an equilibrium phase at ambient pressure. The formation of BaGe5 from Ba8Ge43□3 was traced through microstructure analysis, revealing microdomains with pleochroic behavior in polarized light. © 2024 Acta Materialia Inc.