The generation of hydrogen from water and sunlight through photoelectrochemical cells (PECs) offers a promising approach for producing scalable and sustainable carbon-free energy. However, the design of high-performance PECs requires a detailed understanding of physicochemical properties of the interface between semiconductor photoelectrode materials and liquid water, which is often difficult to probe under operating conditions. In this presentation, we discuss how high-level first-principles simulations can be integrated with advanced experimental characterization techniques to unravel the key chemical and electronic properties of solid/liquid electrochemical interfaces. Specific discussion focuses on the combination of X-ray photoelectron spectroscopy (XPS) simulations and near-ambient-pressure XPS experiments for the identification of solid/liquid interfacial chemical composition and speciation. We also show how first-principles molecular dynamics simulations can be coupled to many-body perturbation theory to directly probe the link between interfacial electronic properties and local chemistry, which is necessary for devising meaningful strategies to improve PEC performance and durability. Finally, we illustrate the benefit of this combined approach towards insights into native oxide chemistry at prototypical InP/water and GaP/water interfaces. We show that GaP exhibits a larger water-induced band edge shift and a broader XPS spectrum than InP, which can be traced to fundamental differences in the surface oxide chemistry by direct comparison between simulations and experiments.This example suggests a more general roadmap for obtaining a realistic and reliable description of the chemistry of complex interfaces by combining state-of-the-art computational and experimental techniques.