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This work demonstrates the potential of thermally actuated micro-electromechanical resonators to be used as highly sensitive and fast sensors distinguishing between different gases and gas mixtures based on their thermo-physical properties. This approach is especially suitable for detection of hydrogen due to its extremely high thermal conductivity compared to other gases. On the other hand, as observed in our experiments, the resonant frequency of thermally actuated silicon micromechanical resonators is very sensitive to the thermal conductivity of its surrounding gases. This is expected to be mainly due to the induced changes in the static temperature of the resonator as the heat transfer coefficient changes with thermal conductivity of the surrounding gas. Frequency shifts as high as 3800ppm where measured for I-shaped resonators when their surrounding gas was changed from air to 100% hydrogen. Frequency deviation of 2200ppm was measured for a self sustained micromechanical thermal-piezoresistive oscillator upon its exposure to a hydrogen-air mixture with 100ppm hydrogen concentration.