The electronic structure of chemisorption complexes on the surface of oxidic α-Sn x WO y - and Cd x S-based gas sensors with different elemental composition were comparatively studied by XPS, CEMS and IR-spectroscopy combined with conductivity measurements. The surface sites of Cd and Sn with different coordination were characterized by chemisorption of CO. It was found, differing from the case in a well-oxidized SnO 2 , that electrical conductance of the Sn x WO y and Cd x S films appeared to either increase or decrease when exposed to CO depended on concentration of the surface Sn and Cd ions, the partial pressure of the analyzed gas and the operating temperature. It is argued that the surface metallic species on the investigated films are responsible for oxidizing and reducing types of chemisorption reactions. The relative location of energy levels, formed in the band gap by external orbitals of the chemisorbed complex to the Fermi energy was considered as the determining factor for the observed different chemisorption mechanisms.