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It is well known that miniaturized gas sensors can be obtained by coating a planar interdigital capacitor with a sensitive layer and exploiting the impedance changes induced by analyte incorporation. Commonly, one assumes that the sensor characteristics only depend on the sensitive layer but not on the substrate carrying the interdigital capacitor. However, in many cases, the sensor has to be exposed to higher temperatures. This is true, for instance, when hot exhaust gases are to be monitored or when the functional principle of the sensitive layer requires heating. A case in point are hydrocarbon sensors based on a zeolite layer in contact with a semiconducting metal-oxide such as Cr2O3. At operating temperatures of several hundred degrees centigrade, the substrate does influence the sensor impedance in a manner that can no longer be neglected. This contribution reports on the model-based interpretation of experimental results which allows one to draw conclusions on this substrate influence. The experiments were carried out with Si, glass, and quartz substrates at 300 degC and in various atmospheres. The current results bear relevance for the design and understanding of all planar interdigital-electrode sensors evaluated by impedance spectroscopy.