Previous electrical characterization of BaTiO3–CuO as a carbon dioxide sensing material demonstrated the enhancement of the sensor response in thin-films with Ag as an additive. In the present work, a detailed characterization is carried out to understand the effects of the Ag concentration variation on the sensor response. Moreover, it is also illustrated that whereas the CO2 sensing process at 300°C is reversible, an inhibition of the sensor occurs at 400°C. DRIFT spectra are obtained to analyze the species adsorbed on the surface of the BaTiO3–CuO in order to explain the sensing chemical reaction. FEG-SEM images show the surface of the different tested layers. TOF-SIMS analysis is carried out in order to get the final distribution of the different materials present in the sensing layer.