They consist of a ceramic substrate heated by wire and coated by a metal oxide semiconducting film. The metal oxide coating may be either of the n-type which responds to oxidizing compounds, or of the p-type which responds to reducing compounds (Mielle, 1996). The film deposition technique divides each sensor type into thin (6–1000 nm) or thick (10–300 μm) film MOS sensors. The first one offers a faster response and significantly higher sensitivity but is much more difficult to manufacture in term of reproducibility. Therefore, commercially available MOS sensors often are based on thick film technologies. Due to the high operating temperature (200– 650 °C), the organic volatiles transferred to the surface of the sensors are totally combusted to carbon dioxide and water, leading to the change in the resistance. MOS sensors are extremely sensitive to ethanol which blinds them to any other volatile compound of interest.