Here we demonstrate full aliovalent substitution of the narrow gap (1.2 eV) layered multiple anion semiconductor Bi4O4SeCl2, a natural heterostructure of BiOCl and Bi2O2Se with possible solar absorber applications , to the wide gap (2.7 eV) semiconductor SrBi3O4Cl3 with applications in catalysis (photocatalytic water splitting and ethane dehydrogenation ). Simultaneous Sr 2+ for Bi 3+ and Cl -for Se 2-substitution is shown to be possible all the way to the Se 2-free compound SrBi3O4Cl3. The aliovalent substitution is associated with the layersegregation of the Se 2-and Sr 2+ ions. Despite the large band gap difference of the two end members (ΔEG = 1.5 eV), the band gap increases modestly with substitution until compositions where two band gap transitions are observed, allowing for the possibility of two photon absorption. The effect of substitution of the alkali earth on the cation site was also investigated, with synthesis and characterisation of the compound CaBi3O4Cl3, demonstrating an increase in band gap and alkali earth independent thickness of the ionic Bi2O2Se type layer, indicating that aspects of the structure are primarily dependent on anion composition. The Bi4O4SeCl2 structure is therefore open to multiple axes of compositional tuning, establishing a paradigm of aliovalent substitution in multiple anion compounds for materials design.