A new sensor for gases is presented. The device is a micro-wave spectrometer. Frequency modulated ,signal,which is proportl4 onal to the microwave cavity resonance profile,is displayed on the oscilloscope. At the same time, this signal is compared with the accurate informations of different gases,measured at the same operating conditions..These informations are charged in a computer interfaced with the oscilloscope. This technique is able to identify the gases of types:linear,diatomic,symmetric, and asymmetric polar and non polar molecules. Experimental and calculated results are illustrated. Accuracy and sensitivity of the system are discussed.