By impedance spectroscopic measurements on gas sensors the real- and i
maginary part of the resistivity is evaluated in the Frequency range o
f 1 Hz to 1 MHZ. A detailed characterization is possible which provide
s much more information about semiconducting gas sensors than the usua
l d.c. resistivity measurement. Equivalent circuits describe formally
different functional components of the sensor. This approach is used t
o describe the gas interaction of the sensor by fit parameters of the
impedance spectra. With this knowledge a systematic improvement of the
design of the sensor is possible in view of achieving better selectiv
ity, sensitivity and stability. This may be achieved by varying the co
ntact geometry, dopants, structure of the sensitive layer, temperature
and operation frequency.