The invention relates to a FET-based detector for carbon monoxide that is based on two sensitive layers (7, 8). The first of said sensitive layers (8) is catalytically active and thus react equally to alcohols, particularly ethanol, and carbon monoxide. The second of said sensitive layers (7) is not catalytically active and thus does not react to carbon monoxide, rather only to ethanol. From the comparison of the signals of both layers, the concentration of carbon monoxide can be determined. The two sensitive layers are implemented via similar metal oxide layers, wherein there is an additional layer having a catalyst such as palladium for the catalytically active layer. Alternatively, two different layers can be used, one of which is already catalytically active without an additional layer.