Computational analysis of the cation mixing thermodynamics confirmed that Sn-and Ti-vacancy-ordered perovskites are highly miscible. From hybrid Density Functional Theory (DFT) calculations of the mixed compositions, it was observed that the mixing enthalpies ΔH mix for the Sn/Ti alloys are extremely low (Figure ); <1 meV/mixed-site. A low energetic cost to cation mixing is to be expected in these vacancy-ordered structures consisting of isolated rigid octahedra, due to the extremely weak interactions between B-site cations (with metalmetal distances on the order of 8 Å) and negligible disruption of M-X bonding mentioned above. As discussed in Ref , the fully-oxidized cations (B +4 compared to B +2 in prototypical ABX 3 perovskites) here have essentially no bonding covalent interactions between B-site cations -aiding for instance, the emergence of ultra-strong excitons -and thus the mixing of isovalent species results in minimal disruption to both ionic and covalent bonding interactions, yielding almost "ideal mixture" behavior for the alloys. Consequently, the configurational entropy contribution ΔS mix dominates the free energy of mixing ΔG mix for these alloyed vacancy-ordered perovskites as shown in Figure , with cation mixing predicted to be thermodynamically favorable (ΔG mix < 0) at essentially all finite temperatures. Notably, this ideal mixing behavior explains and facilitates the recent low-temperature synthesis of high-entropy semiconductors from these vacancy-ordered perovskite compounds.