BaTiO3 and SrTiO3 perovskites of the A(2+)B(4+)O(3) type form complete solid solution, Sr1-x BaxTiO3, which can accommodate a substantial amount of donor dopants, for example, La. At high oxygen partial pressure, La dopants in SrTiO3 are compensated by A-site vacancies, whereas in BaTiO3 they are compensated by B-site vacancies. Therefore, donor compensation in the Sr1-xBaxTiO3 solid solution should demonstrate a crossover from the A-site vacancies at x=0 to the B-site vacancies at x=1. One may expect, therefore, that at some critical concentration, x(c), the free energy of the Sr1-xBaxTiO3 system can become invariant to the vacancy compensation regime. In other words, the system will adopt either A- or B-site vacancies depending on the target chemical composition. Based on the Rietveld refinement of X-ray diffraction patterns and their phase composition analysis as well as scanning electron microscopic and transmission electron microscopic data, we demonstrate that the 28% La-doped Sr1-xBaxTiO3 system equilibrated at 1400 degrees C indeed becomes invariant to the vacancy-type compensation at x(c)approximate to 0.25 and can accommodate A- and B-site vacancies at any given ratio. Finally, we propose a microscopic model based on the off-center Ti displacement and the partial covalency of Ti-O bond to explain the distinct difference in the vacancy compensation mechanisms in BaTiO3 and SrTiO3. These findings are important for a further understanding of the thermodynamics of the intrinsic point defects in perovskites as well as for the improvement of electrical performance of the solid oxide fuel cells, ferroelectric, and voltage-tunable ceramics.