The effect of the alkali earth is seen when comparing the band gaps of SrBi3O4Cl3 with CaBi3O4Cl3, which are both indirect and 2.58 (7) eV and 2.72 (8) eV respectively (Supplementary Figure ). The direct gaps are 2.80(6) and 2.90(4) eV respectively as well. In the DOS calculations (Supplementary Figure ), when Sr is swapped for Ca, the computed energy gap increases from 1.60 to 1.67 eV, an increase that is within error of the experimentally observed increase. In both structures, the valence band edge is dominated by O 2p states and the conduction band by Bi 6p states. The increase in the gap from Sr to Ca is primarily related to the increase in the ionicity of the latter cation. This demonstrates the alkali earth cation to be a minor but independent source of tuning the electronic structure in this series; replacing Sr with the smaller and less electropositive Ca leads to a larger band gap allowing for another mechanism to tune the band gap in this structure type. Overall, the results presented here differentiate this aliovalent anionic substitution from the usual isovalent substitution; by going from one group to another, the energy difference in the orbital energy levels is increased more than by going from e.g. Cl to Br to I. For example, the band gap difference between Bi2O2Se (0.8 eV) and BiOCl (3.5 eV) is much larger than the difference between BiOCl (3.5 eV) and BiOBr (2.9 eV) . Thus, aliovalent anion substitution has qualitatively different effects on the electronic structure compared with isovalent anion substitution. The ability to synthesize this series demonstrate that Bi4O4SeCl2 has significant compositional and structural flexibility, with full aliovalent substitution possible all the way up to SrBi3O4Cl3, and substitution of Ca possible for Sr. The existence of photocatalytic PbBi3O4Cl3 28 as well as analogous bromides and other stacking homologues such as Bi6O6Se2Cl2 indicates that the level of tunability in terms of compositions and properties in this family of multiple anion and layered materials is vast, providing an alternative paradigm for properties design by anion substitution. A generalised formula can be written as AxBi2+2n-xO2+2nSen-xX2+x where A = (Ca, Sr, Pb) and X = (Cl, Br). It is also worth investigating if Bi4O4SeCl2 can be doped aliovalently in the other direction, e.g. MxBi4-xO4Se1+xCl2-x where M is a tetravalent element such as Zr, Hf or possibly Ge/Sn.