Structural investigations of the compounds in the series Ba_0.27Sr_0.73Zr_yTi_1-yO₃ have been carried out. The crystal structure goes to lower symmetry with increasing Zr content and the tolerance factor decreases. The variation of V_a/V_b is linear with the tolerance factor. The tilt system is a^-a^-c⁰ for compositions with y = 0.50 and 0.65. The tilt system is a^-a^-c⁺ for compositions with y = 0.80, 0.90 and 0.97 and for BSZT (Ba_0.29Sr_0.71Zr_0.97Ti_0.03O₃) and BSZTTa (Ba_0.29Sr_0.71Zr_0.95Ti_0.04Ta_0.01O₃). The tilt angle with respect to the pseudocubic [100] and [001] directions increases with the Zr content. For compositions with y = 0, 0.25 and 0.40, the coordination of the A ion (Ba or Sr) is 12, whereas the A-ion coordination number is 11 for compounds with y = 0.50 and 0.65 and it is lower for compositions with y = 0.80, 0.90 and 0.97 and for the compounds BSZT and BSZTTa. The decrease in coordination number indicates that the compounds are becoming more and more covalent with increasing Zr content. A graph of |90°-_pc| against the tolerance factor shows that |90°-_pc| falls sharply at a tolerance factor corresponding to the compositions used in the fabrication of dielectric resonators. TC_f values show that the sign of this parameter changes at a tolerance factor near 0.965 and has a minimum for compounds with orthorhombic Pbnm/Pnma structures.