The design of optimal interfaces between photoelectrodes and catalysts is a key challenge in building photoelectrochemical cells to split water. Iridium,dioxide (IrO2) is an,efficient catalyst for oxygen evolution, stable in acidic conditions, and hence a good candidate, to be. interfaced with photoanodes. Using first-principles quantum mechanical calculations; we investigated the structural and electronic properties of tungsten trioxide (WO3) surfaces interfaced with an IrO2 thin film. We, built a microacopic model of the interface that exhibits a formation energy lower than the surface energy of the most, stable IrO2 surface, in spite of a large lattice Mismatch, and has to impurity states pinning the Fermi.,level. We found that upon full coverage of WO3 by IrO2, the two oxides form undesirable Ohmic contacts. However, our calculatious predicted that if both oxides are partially exposed to water solvent, the relative position of the absorber conduction band and the catalyst Fermi level favors charge transfer to the catalyst and hence water We propose :that, for oxide photoelectrodes interfaced with IrO2, it is advantageous to form tough interfaces with the catalyst, e.g., by depositing nanoparticles, instead of sharp interfaces with thin films.