The development of surface characterization techniques can contribute to significant progress in cutting-edge science regarding sustainable energy conversion and environment-friendly technology at the molecular level. In the last decade, the advanced operando observation tools have been widely utilized in structural, compositional, and thermodynamic behavior observations at the gas-solid interface in surface science and heterogeneous catalysis. In particular, the synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) plays an important role in fundamental investigations of complicated reactions to reveal physicochemical properties on the material surface under the more realistic condition. In this thesis, I will introduce technical details on the hemispherical electron analyzer equipped with the differential pumping system to build high-flux photon delivery into the analysis chamber at elevated pressure conditions. By using AP-XPS, surface chemical states and interfacial structures have been studied on representative metal oxide model systems such as Ga2O3-x, SrTiO₃ (001), and SrRuO₃/SrTiO₃ (001). The core-level spectra analysis results highlight that the surface redox process and the variation of bandgap on the Ga2O3-x during post-annealing process, the nature of electron depletion and band bending with vacancy formation on the SrTiO₃ (001) substrate, and the influence of surface migration of lattice oxygen process at the SrRuO₃-SrTiO₃ interface on the structural phase transition. Those observed phenomena at the surface show that the bunch of reactive molecules' interaction at the gas-solid interface could affect the surface electronic structure near the Fermi-edge on the condensed matter, which provides more reliable spectroscopic evidence for understanding energy conversion issues on the surface in the real world.