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We ?report the microstructure and gas-sensing properties of the SnO2–TiO2 ?composite oxide dope ?with Ag ?ion ? prepared by ?the sol–gel method. Of ?all ?various volatile organic compounds ?(VOCs) ??such as ethanol, ?methanol, ?acetone ?and ?formaldehyde ?were ?examined, ?the ?sensor ?exhibits ??remarkable selectivity to each VOCs at different operating temperature. Further investigations ?based on ?quantum chemistry calculation show that difference orbital energy of VOCs molecule may be??a qualitative factor to affect the selectivity of the sensor.