Gas sensors that are based on metal oxides are extensively used to detect gaseous compounds in many different applications. One of the main tasks for improving the sensor performances is to understand the mechanism at the base of the sensing properties for each specific material. In this work, pure and mixed oxides were selected and synthesized in the form of nanometric powders. They were characterized by spectroscopic techniques, i.e., absorbance FT–IR and diffuse reflectance UV–Vis–NIR spectroscopies, to obtain information about the electronic properties and the type of defects that are involved at the root of the gas-sensing capabilities. The electrical characterization and the gas-sensing measurements were carried out on the related thick films. Finally, for each material, a description of the specific sensing mechanism is proposed by combining the characterization results.