Photocatalysis utilizes the energy delivered by light and enables chemical reactions that otherwise cannot take place. When used to power thermodynamically uphill reactions, photocatalysis offers a solution to large-scale solar energy storage. Despite over four decades of intense research, however, photocatalysis remains either too expensive or too inefficient or both. Poor understanding of the mechanisms behind the low performance is a key reason that limits the progress of this important field. To address this critical challenge, and to complement existing efforts focused on discovering new materials for photocatalysis, we present here a series of experiments designed to elucidate the working principles of photocatalysis. First, we carried out measurements of the system under quasi-equilibrium conditions, and the goal was to understand how the charge separation capability is influenced by the surface treatments of the photocatalyst. Next, we conducted intensity modulated photocurrent spectroscopy and established a quantitative correlation between surface treatment and charge transfer kinetics. Afterward, we compared two co-catalyst systems, one heterogeneous and one homogeneous and found that they improve the performance of the photocatalyst by fundamentally different mechanisms. Lastly, we employed synchrotron X-ray spectroscopy to provide a complete picture of surface energy evolution as a function of water splitting history. The key value generated by this body of research lies in the fundamental understanding of the thermodynamics and kinetics of the photocatalyst/electrolyte interface, which is expected to serve as a foundation for future research on photocatalysis.