BaCuSi2O6 is a quasi-two-dimensional spin dimer system and a model material for studying Bose–Einstein condensation (BEC) of magnons in high magnetic fields. The new Ba1–xSrxCuSi2O6 mixed system, which can be grown with x ≤ 0.3, and BaCuSi2O6, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure (I41/acd) at room temperature. The mixed system shows no structural phase transition so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as a control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure.