The lanthanum-based materials, due to their layered structure and f-electron configuration, are relevant for electrochemical application. Particularly, La 2 O 2 CO 3 shows a prominent chemoresistive response to CO 2 . However, surprisingly less is known about its atomic and electronic structure and electrochemically significant sites and therefore, its structure-functions relationships have yet to be established. Here we determine the position of the different constituents within the unit cell of monoclinic La 2 O 2 CO 3 and use this information to interpret in situ high-energy resolution fluorescencedetected (HERFD) X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectroscopy (vtc XES). Compared with La(OH) 3 or previously known hexagonal La 2 O 2 CO 3 structures, La in the monoclinic unit cell has a much lower number of neighboring oxygen atoms, which is manifested in the whiteline broadening in XANES spectra. Such a superior sensitivity to subtle changes is given by HERFD method, which is essential for in situ studying of the interaction with CO 2 . Here, we study La 2 O 2 CO 3 -based sensors in real operando conditions at 250°C in the presence of oxygen and water vapors. We identify that the distribution of unoccupied La d-states and occupied O p-and La d-states changes during CO 2 chemoresistive sensing of La 2 O 2 CO 3 . The correlation between these spectroscopic findings with electrical resistance measurements leads to a more comprehensive understanding of the selective adsorption at La site and may enable the design of new materials for CO 2 electrochemical applications.C O 2 has become a challenge for our society and we have to develop new materials for its photo/electrocatalysis, chemoresistive sensing, and storage (1-8). Particularly, for the variety of electrochemical applications the selective interaction of CO 2 and charge transfer with solids is in the foreground. At the same time, the interaction of CO 2 with solids in the electrochemical cell or sensing device is rather complex, thus it remains challenging to experimentally identify the key elements determining their selectivity and efficiency. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) provide complementary information on the electronic structure of materials (9, 10) and on the orbitals participating in the interaction with absorbing molecules (11). High-energy resolution fluorescence-detected (HERFD) XAS probes unoccupied states with a spectral resolution higher than regular XAS. Furthermore, with the same experimental setup XES can be measured, which allows one to probe the occupied states within the valence band (12). In situ HERFD XAS or XES experiments have been previously carried out to study the catalytic reaction at the surface of noble metals (11, 13-16), zeolites (17), and metal organic frameworks (18). Thus far, no such in situ experiments have been performed to directly track the changes of the electronic structure of a solid and its electrochemical activity toward CO 2...