The sun offers an immense amount of free and clean power, which we have not yet begun to seriously exploit. In addition to directly converting sunlight to electrical energy, we could also generate storable chemical energy, to be used at a time of our choosing. The photoelectrochemical (PEC) decomposition of water to form hydrogen and oxygen is ideal for this purpose, since hydrogen can be generated, stored, and later converted to emission-free electricity using fuel cell technologies. The missing link in this idealized cycle is the first step: an efficient and affordable way to generate hydrogen from water and sunlight. The challenge is to find a low band-gap semiconductor that is stable in solution, catalytically active, and has high overall solar-to-hydrogen conversion efficiency. Metal oxides are ideal candidate photocatalyts because of their low cost and high stability in aqueous solutions.1 Unfortunately their optical band-gaps generally lie outside the visible range (> 3eV), making them good for transparent electronics,2 but bad for solar applications. Despite immense effort, attempts to reduce the band-gaps of catalytically active oxides such as TiO2 through doping or co-doping have not produced a promising material. We must therefore find new avenues of oxide design. Millions of potential multi-ternary oxides exist, but researchers need a strategy to identify the most likely material classes. The valence bands of most oxides generally lie well below the water oxidation potential, while the conduction bands straddle the hydrogen reduction potential. Thus, the task requires raising the valence band, not lowering the conduction band. One approach could harness low binding energy cations with an nd10 electron configuration, such as Cu and Ag. Alternatively, scientists could use low binding energy ns2 cations such as Sn(II), Sb(III), Pb(II), Bi(III). We are currently focusing on the latter. Typical metal oxides such as MgO and ZnO have band edges formed from occupied O 2p and unoccupied cation s orbitals, respectively. Heavier post-transition metal cations are unique because the binding energy of the cation s orbital increases, giving rise to two accessible oxidation states. These orbitals are either nominally filled or empty, such as Sn(II)O/Sn(IV)O2 and Sb(III)2O3/Sb(V)2O5. The filled orbitals were historically thought chemically inert, but stereochemically active. However, we recently showed that cation s − anion p hybridization dominates in these systems and second order cation p coupling with the antibonding cation s − anion p states occurs towards the top of the valence band.3–7 We also verified this theoretical understanding with hard x-ray photoemission spectroscopy.8 Bismuth vanadate (BiVO4) is a ternary oxide combining Bi(III) 6s2 and V(V) 3d0 cations, as shown in Figure 1. It has a welldemonstrated potential for water photodecomposition with the presence of both a low band-gap (2.4eV) and reasonable catalytic activity.1 We used density functional theory to analyze its electronic structure and explain its unusual properties.9 BiVO4 has a direct fundamental band-gap, with the Bi 6s states helping to raise the valence band by 0.4eV, while the conduction band remains low because of the V 3d states. Both the electron and hole effective masses are relatively light, indicating the potential for both p and n-type conductivity. The combination of ns2 and nd0 cations generally results in unique crystal structures because the ns2 ones have a strong preference for disordered coordination that varies with the cation s binding energy. This is demonstrated by BiVO4 , which has distorted 2D stacking of Bi−O−V layers with C6 2h symmetry. The generality of this cation combination is apparent from a recent optical evaluation of ternary tungstenates by