In the ternary system Sr-Cu-Ge, a clathrate type-I phase, Sr 8 Cu 5.3 Ge 40.7 (a = 1.06311(3), exists close to the Zintl limit in a small temperature interval. Sr 8 Cu 5.3 Ge 40.7 decomposes eutectoidally on cooling at 730°C into (Ge), SrGe 2 and 1 -SrCu 2-x Ge 2+x . Phase equilibria at 700°C have been established for the Ge rich part and are characterized by the appearance of only one ternary compound, 1 -SrCu 2-x Ge 2+x , which crystallizes with the ThCr 2 Si 2 structure type and forms a homogeneity range up to x=0.4 (a = 0.42850(4), c = 1.0370(1) nm).Additionally, the extent of the clathrate type-I solid solution Ba 8-x Sr x Cu y Ge 46-y (5.2 y 5.4) has been studied at various temperatures. The clathrate type-I crystal structure (space group n Pm3 ) has been proven by X-ray single crystal diffraction on two single crystals with composition Sr 8 Cu 5.3 Ge 40.7 (a = 1.06368(2) nm) and Ba 4.9 Sr 3.1 Cu 5.3 Ge 40.7 (a = 1.06748(2) nm) measured at 300, 200 and 100 K. From the temperature dependency of the lattice parameters and the atomic displacement parameters, the thermal expansion coefficients, the Debye-and Einstein-temperatures and the speed of sound have been determined.