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.