The precursors of SnO 2 or In 2 O 3 /SnO 2 nanocrystlline powders have been prepared by the sol-precipitation method. The precursors were calcined at different temperatures to prepare SnO 2 or In 2 O 3 /SnO 2 nanocrystalline powders with different particle sizes. The nanocrystallites were examined by differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). And then thick film CO sensors were fabricated using prepared SnO 2 or In 2 O 3 /SnO 2

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... ystlline powders loaded with PdO x . The composition that gave the highest sensitivity for CO was in the weight% ratio of 5 wt.% In 2 O 3 /SnO 2 :PdO x as 99:1(wt %). The composite material was found sensitive against CO at the working temperature 200 °C. It was found that the sensors based on In 2 O 3 /SnO 2 nanocrystalline system exhibited very short response time to CO at ppm level. These characteristics make the sensor to be a promising candidate for detecting low concentrations of CO.