To gain knowledge about the transduction mechanisms involved in the sensitivity of field effect gas sensors towards non-hydrogen containing substances, such as O-2, NO and CO, the sensor signal characteristics during exposure of some conceptually different model sensors to these as well as hydrogen containing gases have been investigated. The MOS capacitor based model sensors employ different combinations of insulator and contact materials, such as MgO, LaF3, IrO2 etc. The gas composition downstream of the sensor during test gas exposure at various conditions has also been studied by mass spectrometry (MS) and compared for the different model systems. The results have been compared to the characteristics of ordinary SiC/SiO2/Pt structures and from the information obtained a tailor made field effect sensor structure for oxygen sensing, to our knowledge for the first time with minimal interference from H-2, CO, and hydrocarbons, has been tested with promising results.