Oxygen interaction of CdS-based gas sensors with different stoichiometric compositions was studied by X-ray photoelectron spectroscopy (XPS) and thermal-desorption mass spectroscopy observations combined with conductivity measurements. Three different types of film with different atomic ratios of Cd/S were used in the study of oxygen chemisorption in a low-temperature range between 300 and 570 K. Spray pyrolysis was used for the deposition of the films on glass substrates; XPS and energy dispersive spectroscopy of X-ray together with atomic force microscopy and scanning electron microscopy were used for the characterization of the composition and structure of the films. It was possible to distinguish different temperature ranges for the thermal desorption of the different adsorbed oxygen species. A comparison of the data from XPS and mass-spectroscopy observations with conductance measurements allows to conclude that metallic surface atoms are responsible for different interactions of oxygen species with CdS surfaces.