Ambient pressure Electrochemical XPS (E-XPS) is used to study surface/gas reactions in electrolyzer materials and electro-catalysts for water splitting and other oxygen reduction reactions, close to operating conditions of temperature and pressure). E-XPS measurements reveal 1) the chemical identity of the adsorbed reaction inter-mediates, and 2) the chemical state of the active materials. This important information is obtained simultaneously with traditional electrochemical characterization (voltammetry, impedance spectroscopy). A full characterization of surface electrochemistry requires knowledge of the composition and chemical state of the electro-catalyst/electrolyte surfaces. Electrochemical XPS provides this information, which is particularly important for transition-metal oxides such as perovskite catalysts, iron oxide phases, ceria (CeO2-x) and other oxide electrocatalysts.  Traditional XPS can only be used in ultra-high vacuum conditions, allowing only ex-situ hydrogenation experiments and subsequent XPS measurements in vacuum. But the chemical reactivity and surface species can vary greatly with and without gas pressure. Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) provides this information. In-situ studies with NAP-XPS can reveal the complex coupling between transport, surface reaction rates, and oxidation state changes.