The defect-pyrochlore-structured photocatalyst CsTaWO6 is an ideal starting material for anion doping from the gas phase, and is known to be highly active for solar hydrogen generation under simulated sunlight without co-catalysts. To investigate the active site of CsTaWO6 for hydrogen generation and to understand the effects of the two d(0) elements in the compound, systematic and successive element substitution of tantalum and tungsten on the crystallographic 16c sites of the starting material has been performed. Substituting lattice tantalum with niobium hardly changes the band gap of the resulting compounds CsTa(1-x)NbxWO6, but the photocatalytic activity for hydrogen generation and oxidation reactions is strongly influenced. By investigating the surface reactivity toward adsorption, surface effects altering the activity are identified. In contrast, substituting lattice tungsten with molybdenum reduces the band gap of CsTaWO6 into the visible-light range. Materials containing Mo are however not able to generate hydrogen anymore, due to the altered conduction band positions proven by density functional theory calculations. CsTaMoO6 exhibits a band gap of 2.9 eV and evolves oxygen efficiently under UV light irradiation after CoPi co-catalyst deposition, and even under visible light small amounts of oxygen.