The development of solution-processed optoelectronic materials based on abundant and non-toxic elements is of great interest for sustainable technological progress. In particular, lead halide perovskites are at present the best performing solution-processed photovoltaic materials, with a power conversion efficiency record of 26% in 2023. Nevertheless, both the regulatory concerns due the presence of lead and poor stability under ambient operating conditions pose major barriers to their commercialization. Lead is toxic and when released from the perovskite structure into the soil it is 10 times more bioavailable than other natural lead sources. The exceptional optoelectronic properties of lead halide perovskites, along with these stability and toxicity drawbacks, have spurred major research interest in 'perovskite-inspired materials'. These are materials with similar structural and/or chemical properties to the prototypical halide perovskites, but avoiding the presence of lead. Within the perovskite-structured subclass of these materials, ionic substitution is employed as a design principle, replacing Pb with a combination of chemically-similar cations. Among these novel lead-free perovskites developed over the last decade, titanium-based vacancy-ordered double perovskites (VODPs)(Cs 2 TiX 6 ; X = I, Br, Cl) have attracted particular attention following the recent demonstration of high performance in solar cell devices, with 3.3% power conversion efficiency -the current best performance for a Sn-and Pb-free perovskite structure. The elemental abundance of Ti and its low toxicity 10 make it a viable replacement for Pb. Beyond photovoltaic applications, the A 2 BX 6 structural family exhibit features associated with both zerodimensional and three-dimensional materials (such as low thermal conductivity, high compressibility and strong exciton binding despite relatively small electronic band gaps), makes them potential candidates for a range of alternative applications, such as potential thermoelectric materials, white-light emitters / phosphors, photocatalysts, high-entropy semiconductors and more, as well as offering a highly-tunable playground to study physical phenomena associated with strong Frenkel excitons. Moreover, these materials have been reported to show non-linear optical activity, being in fact the only known third harmonic generation (THG) active lead-free perovskite structures. The stability of titanium-based vacancy-ordered perovskites has attracted significant debate. While the chloride (Cs 2 TiCl 6 ) has been reported to show slow decomposition in air, only detectable after a couple of weeks, for the bromide (Cs 2 TiBr 6 ) and the iodide (Cs 2 TiI 6 ) compositions, there is less consensus. Chen et al. reported Cs 2 TiBr 6 thin films to show stability in air, while Mendes et al. detected amorphous titanium oxide and cesium bromide on the surface of nascent large-grain Cs 2 TiBr 6 samples. Kong et al., Euvrard et al. and He et al. found moderate to fast decomposition rates when synthesized in powder form, and more recently Grandhi et al. and Liga et al. reported Cs 2 TiBr 6 nanocrystal (NC) films to decompose within hours to minutes. Regarding Cs 2 TiI 6 , while Ju et al. found that it is stable in powder form, Kupfer et al. found the powder to be unstable and Liga et al. observed fast decomposition of the NC films in air. The high instability in air for Tibased vacancy-ordered perovskites significantly hinders the characterization of their properties (particularly for techniques which require contact with air) as well as their application in devices. On the other hand, the Cs 2 SnX 6 ; X = I, Br, Cl family have been shown to be mostly stable in air and under thermal, water and photo-stresses. In fact, these compounds are the decomposition products typically witnessed for the unstable tin(II)-based ABX 3 single perovskites (CsSnX 3 ; X = I, Br, Cl). Consequently, this subclass has received much more research attention over the past few years, particularly when it comes to device applications. The iodide (Cs 2 SnI 6 ) has been found to show some instability under air, though with relatively slow decomposition, as Saporov et al. reported cesium iodide (CsI) peaks appearing in x-ray diffraction (XRD) measurements after 60 days in air. Since Sn and Ti have comparable ionic radii, we hypothesized that mixing tin with titanium in the same host perovskite-like structure could stabilize the final mixed composition. Thus, we prepared mixed compositions of tin-titanium bromide and iodide VODPs (Cs 2 Ti 1-x Sn x X 6 ; X = I, Br) across the entire mixing range x, and studied their properties. We synthesized all the mixed compositions as colloidal nanocrystals (NCs) using the hotinjection method. We employed a combined approach, characterizing the properties of these materials both computationally and experimentally to elucidate the behavior and impact of cation mixing on the stability and optoelectronic properties of this series of novel vacancy-ordered double perovskites. While not the focus of this study, we note that the pure Sn and Ti chloride VODPs were also synthesized and characterized in our previous work, and so the same computational analysis was performed for Cs 2 Ti 1-x Sn x Cl 6 -appended to the SI (Figures &).