The defect density of a material is central to its properties. Here, we show, employing EXAFS measurements and MD simulation, how the Ba-Li antisite defect density of perovskite-structured BaLiF3 nanoparticles can be tuned. In particular, we show that ball milling reduces the defect content. Conversely, thermal annealing increases the defect density. The work represents a first step toward tailoring the properties of a material via defect tuning postsynthesis.