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Resistance characteristics of thin film sensors based on uncoated SnO2, SnO2 with CuO overlayer and SnO2 with CuO dotted clusters are compared in three different backgrounds of air, oxygen and vacuum. Measurements for the three sensor configurations are carried out as a function of temperature. The novel dispersal method of CuO catalyst in the form of dotted clusters is seen to enhance the oxygen adsorption activity on surface of SnO2 thin film sensors. Conversion of molecular oxygen (O2 -) to atomic oxygen (O-) is shown to reduce the concentration of charge carriers in the conduction band of SnO2 film. Co-existence of a greater amount of adsorbed oxygen on the SnO2 film surface in conjunction with modulation of the space-charge region at the CuO-SnO2 interface are attributed to influence the resistance of the sensor structures under reducing gas.