Porous silicon oxycarbide (SiCO) has unique gas sensing properties at various temperatures. Characterizing the gas sensing mechanism is important for evaluating and designing porous silicon oxycarbide gas sensors. In this work, the temperature-dependent gas sensing properties of porous silicon oxycarbide are investigated. At 300 K, porous SiCO is significantly more sensitive to NO2 than CO, H-2 and acetone. However, at 773 K, SiCO is less sensitive to NO2 compared to H-2. An O-H bond with a length of 1.04 angstrom forms between H-2 and the mixed Si-C/O bond, and the incorporation of H-2 results in an obvious increase in the density of states near the Fermi level. Therefore, porous SiCO is highly selective and sensitive towards H-2 at high temperatures, and its response to NO2 disappears, which is consistent with the experimental conclusions. Moreover, our results show that the mixed SiC/O bonds are the primary sensing sites for NO2 (at 300 K) and H-2 (at 773 K). The proposed theoretical approach can be used to evaluate and predict the temperature-dependent gas sensing properties of porous SiCO.