Abstract <p>The influence of oxygen chemisorption on the electrical and optical properties of gas-sensitive nanocrystalline SnO₂ and ZnO films is studied by measuring the temperature dependences of thermally stimulated conductivity kinetics, thermoelectric power, photoconductivity spectra, reflection, and transmission in the temperature range of 300–800 K. Extrema are observed in the obtained temperature dependences, and their origin is explained by the chemisorption of various oxygen species using a generalized equation of equilibrium activated adsorption. The regions of maximum gas sensitivity coincide in temperature with the peaks of oxygen chemisorption.</p>