While photocatalytic water-splitting is a promising alternative energy source, low photocatalytic efficiencies in the visible spectrum hinders its widespread deployment and commercialization. Although screening combinations of new materials and characterizing their reaction kinetics offers possible improvements to efficiency, current experiments are challenged by expensive bulky setups and slow recovery of particles downstream. Optofluidics is a good platform for screening Z-scheme catalysts cheaply and rapidly. By alleviating the problems of mass transport it can also potentially increase reaction rates and efficiencies. Here, we demonstrate a novel optofluidic device based on applying catalyst sol-gels on planar channels while measuring the reaction output by monitoring the depletion of the redox mediators. We use our setup to study the kinetics of the TiO(2)-Pt water-splitting reaction mediated by I(-)/IO(3)(-) redox pairs under different flow rates. In particular, for TiO(2)-Pt, we show ~2-fold improvements in reaction rates and efficiencies.