The chemical gas sensor area has gained large improvements from the

nanoscience technology, e.g., more reproducible processing including control

on the nanoscale of annealing procedures, which implies improved long-term

stability. Analytical tools have developed towards detection and investigation

of ever smaller size phenomena. This has been of importance for the key

problem of chemical gas sensors, the detailed understanding on the nanoscale

level of the gas-sensing mechanism. In this chapter, which deals with FET (field

effect transistor) gas sensor devices, we will review analytical tools that provide

information about the detection mechanism with special emphasise on the FET

sensor area. The DRIFT, diffuse reflection infrared Fourier transform, spectroscopy

as a rather new and very important tool is reviewed. Theoretical modelling

will speed up the process to provide further details in the mechanistic

studies. Examples will be given in this chapter. A number of other important

analytical tools will be briefly described.