The oxygen controlled chemoresistance of tin oxide based gas sensors was investigated in the temperature range from 20 to 320 degrees C. Polycrystalline thin film gas sensors were fabricated by the reactive dc-magnetron sputtering. The parameters of the sensors were also modified by additional Pt or Sb doping. The effect of surface oxygen species on the chemoresistance was studied by x-ray photoelectron spectroscopic (XPS) analysis of the surface chemical composition before and after different sample treatments. The oxygen peak at a binding energy 531.8 eV in the XPS core level spectrum was found to be related to the chemisorbed molecular oxygen O-2(-). The variation of an amount of the O-2(-) species is found to be the main cause of gas sensitivity of tin oxide films at temperatures from 20 to 230 degrees C.