Effect of Co-doping on gas sensing, electrophysical and structural properties of the SnO 2 films deposited by spray pyrolysis has been studied. It is found that the influence of Co-doping on parameters of the SnO 2 -based gas sensors depends on the concentration of doping additives and could be accompanied by either improvement of sensor parameters at low levels of doping ( C Co < 2-4 %) or degradation of the gas sensor operation characteristics while the concentration of additives exceeds 2-4%. An explanation of observed effects is given.