Hydrogen produced by sunlight is a very promising, environmentally-friendly energy source as an alternative for increasingly scarce and polluting fossil fuels. Since the discovery of hydrogen production by photocatalytic water dissociation on a titanium dioxide (TiO2) electrode 40 years ago, much research has been aimed at increasing the process efficiency. Remarkably, insights into how water is bound to the catalyst and into the dynamics of the photodissociation reaction, have been scarce up to now, due to the lack of suitable techniques to interrogate water at the interface. The aim of this proposal is to provide these insights by looking at specifically the molecules at the interface, before, during and after their photo-reaction. With the surface sensitive spectroscopic technique sum-frequency generation (SFG) we can determine binding motifs of the ~monolayer of water at the interface, quantify the heterogeneity of the water molecules at the interface and follow changes in water molecular structure and dynamics at the interface during the reaction. The structure of interfacial water will be studied using steady-state SFG; the dynamics of the water photodissociation will be investigated using pump-SFG probe spectroscopy. At variable delay times after the pump pulse the probe pulses will interrogate the interface and detect the reaction intermediates and products. Thanks to recent developments of SFG it should now be possible to determine the structure of water at the TiO2 interface and to unravel the dynamics of the photodissocation process. These insights will allow us to relate the interfacial TiO2-water structure and dynamics to reactivity of the photocatalyst, and to bridge the gap between the fundamentals of the process at the molecular level to the efficiency of the photocatalys. The results will be essential for developing cheaper and more efficient photocatalysts for the production of hydrogen.