Modifying the atomic and electronic structure of materials by chemical substitution is a common method of achieving properties by design. Cations and metal atoms are the most frequent choices for chemical substitution; replacing anions with ones from a different chemical group is unusual due to the very different orbital energies and electronegativities involved. Here we demonstrate full substitution of Se by Cl in the visible band gap material Bi₄O₄SeCl₂charge balanced by simultaneous replacement of Bi with Sr, all the way to the wide gap photocatalyst material SrBi₃O₄Cl₃. This compositional flexibility is associated with the layer-segregation of Sr and Se atoms. The crystal structure and electronic structure change non-linearly, with a compositional regime of two band gap transitions observed, due to the introduction of in-gap Se states to the electronic structure. The material CaBi₃O₄Cl₃ is also synthesized, revealing the separate effects on the crystal structure of the anion and cation composition. This work presents aliovalent anion substitution in multiple anion materials as a strategy for tuning between narrow and wide gap materials, with properties showing more than one optical transition achievable at intermediate compositions.