The adsorption of a gas on the surface of a semiconducting oxide can induce a significant change in the electrical resistance of the material. This effect is at the basis of the development of chemiresistors for gas detection. Due to their high sensitivity, tunable selectivity, easy production, small dimensions and low cost, they are successfully used in a broad range of applications (pollutant monitoring, food quality control, industrial systems control, medical diagnosis) to detect a large number of gaseous compounds. Despite this, an increasing demand of gas sensors with high performances has been documented. Many actions can be made to improve the sensing performances, such as the synthesis of nanostructures with high specific surface area, the loading with noble metals, but the first issue is to understand the sensing mechanism of the materials and its sensing properties. This work is aimed to determine the sensing mechanism for a variety of semiconducting oxides (single, doped or combined) correlating them with the sensing performances of the sensors. The functional materials (SnO₂ MoO3, WO3, NiO, ZnO, TiO₂, W-Sn mixed oxide, etc.) were prepared and characterized by means of spectroscopic techniques (absorbance FT-IR, diffuse reflectance UV-Vis-NIR) to shed light on the electronic properties and defects involved at the roots of the sensing capability. The spectroscopic responses were studied both for the interaction with pure gases and for mixture pollutant/O2 at different concentrations. Furthermore, the functional materials we deposited on alumina substrates to obtain thick films for electrical characterization and gas sensing measurement. Finally, from the cross analysis of the results a description of the specific sensing mechanism is proposed for each material.