Two materials with similar structures but differing band gaps, SrBi3O4Cl3 and Bi4O4SeCl2, can be tuned between each other across the complex solid solution series SrxBi4-xO4Se1-xCl2+x. Further, the alkali earth cation can be replaced by Ca, demonstrating this family of multiple anion Bi-based layered materials to exhibit a large amount of chemical flexibility. Due to the very different electronegativities, any amount of Se for Cl substitution leads to a collapse of the band gap by about 1.2 eV, and at small amounts of Se, two band-gap type transitions are observed in the diffuse reflectance. At larger amounts of Se, a small but linear change in band gap is observed with composition. The structure behaves as more complex than a typical solid solution, with the presence of Sr affecting the distribution of Se across the anion sites. Overall, these results demonstrate that aliovalent anion substitution can be done across a wide range of composition for multiple anion layered materials, and leads to a tuning of the electronic structure and crystal structure that is complex but still follows chemical rules, leading to different types of properties from either aliovalent cation substitution or isovalent anion substitution. Aliovalent anion substitution is therefore a complementary approach to structural and electronic tuning, particularly in layered multiple anion solids.