A thorough analytical electron microscopy investigation into the oxidized nickel cocatalyst structure on known photocatalytic materials NiTa2O6 and InTaO4 has been conducted. These photocatalyst materials have previously been reported in the literature to have activity in the splitting of water to form H-2, and with the addition of a cocatalyst, such as NiO/Ni core shell nanoparticles, activity has been suggested to improve. Although important implications are placed on the effect of this cocatalyst, its structure has not previously been examined in detail. By using high resolution analytical transmission electron microscopy it has been determined that the structure is not as simple as suggested and, although a core shell metal/ metal oxide structure is produced, in neither case is a perfect Ni/NiO composition obtained. In particular, in the case of the InTaO4 based material, the nominal NiO/Ni cocatalyst contains indium diffused into the nanoparticles from the photocatalyst support. The general implications of this result for the design of potential photocatalyst systems are discussed.