Automated systems for electrochemical synthesis and high throughput screening of photoelectrochemical materials were developed and used to prepare tungsten-molybdenum mixed metal oxides, WO3-MoO3. Two-dimensional arrays (library) of diverse composition were synthesized by automated cathodic electrodeposition on Ti foil substrate. Electrolytes were prepared by mixing W-peroxo complex and Mo-peroxo complex. The compositions in films were found to strongly depend on the composition of electrolyte, and found to be relatively independent of deposition voltage. The morphology was found to be very homogeneous. The electrodeposited oxides typically showed n-type behavior. Automated measurement of photocurrent by using a scanning photoelectrochemical cell showed that mixed oxides showed higher photocatalytic activity than pure tungsten oxide or pure molybdenum oxide. Photoactitvities of the mixed oxides were strongly dependent on the film composition.