Lead-based perovskites have achieved excellent photovoltaic efficiencies in the last decade, but key intrinsic issues related to their instability and Pb toxicity need to be overcome for their successful commercialization. In this combined experimental–computational study, we investigate the structural and optoelectronic properties of the novel vacancy-ordered lead-free perovskites (CH3NH3)3(Bi1–xSbx)2I9. We find complete miscibility across the solid solution with less than 1% change in the lattice parameters. This miscibility extends to the full configurational disorder between Bi and Sb and a subsequent reduction in the experimentally observed photoluminescence quantum yields. We highlight the significance of the observed band-gap bowing as a means to fine-tune the electronic structure for optoelectronic devices. The substitution of Bi with Sb leads to lower calculated electron and hole effective masses that result in lower exciton binding energies. We show a clear shift from a strongly bound to a weakly bound excitonic regime with the substitution of Bi with Sb, which correlates with the increase in device performance.