Nanocrystalline Sr_1−xBa_xSnO₃ (x=0, 0.2, 0.4, 0.8, 1) perovskite photocatalysts were prepared by microwave synthesis in an ionic liquid (IL) and subsequent heat-treatment. The influence of the Sr/Ba substitution on the structure, crystallization, morphology, and photocatalytic efficiency was investigated and the samples were fully characterized. On the basis of X-ray diffraction results, as the Ba content in the SrSnO₃ lattice increases, a symmetry increase was observed from the orthorhombic perovskite structure for SrSnO₃ to the cubic BaSnO₃ structure. The analysis of the sample morphology by SEM reveals that the Sr_1−xBa_xSnO₃ samples favor the formation of nanorods (500 nm–5 μm in diameter and several micrometers long). The photophysical properties were examined by UV/Vis diffuse reflectance spectroscopy. The band gap decreases from 3.85 to 3.19 eV with increasing Ba²⁺ content. Furthermore, the photocatalytic properties were evaluated for the hydroxylation of terephthalic acid (TA). The order of the activities for TA hydroxylation was Sr_0.8Ba_0.2SnO₃>SrSnO₃>BaSnO₃>Sr_0.6Ba_0.4SnO₃>Sr_0.2Ba_0.8SnO₃. The highest photocatalytic activity was observed for Sr_0.8Ba_0.2SnO₃, and this can be attributed to the synergistic impacts of the modification of the crystal structure and morphology, the relatively large surface area associated with the small crystallite size, and the suitable band gap and band-edge position.