Bismuth-based perovskites are of interest as safer alternatives to lead-based optoelectronic materials. Prior studies have reported on the compounds Cs3Bi2Cl9, Cs3Bi2I9, and Cs3Bi2Cl3I6. Here we examine a range of compounds of the formula Cs3Bi2(Cl1-xIx)9, where x takes values from 0.09 to 0.52. Powder and single-crystal X-ray diffraction were used to determine that all of these compounds adopt the layered vacancy-ordered perovskite structure observed for Cs3Bi2Cl3I6, which is also the high-temperature phase of Cs3Bi2Cl9. We find that, even with very small iodine incorporation, the structure is switched to that of Cs3Bi2Cl3I6, with I atoms displaying a distinct preference for the capping sites on the BiX6 octahedra. Optical absorption spectroscopy was employed to study the evolution of optical properties of these materials, and this is complemented by density functional theory electronic structure calculations. Three main absorption features were observed for these compounds, and with increasing x, the lowest-energy features are red-shifted.