The use of an unmodified variable gap micromachined silicon differential pressure sensor as an electrically tunable interferometer of Perot and Fabry for the detection of gases having spectral absorption bands in the infrared domain of wavelength between 2 and 5 μm is described. The pressure sensor comprises three wafers separated by two air gaps and is analyzed to show its suitability to realize an electrically tunable filter for gas detection. The experimental spectral transmissions of the sensor, obtained with a Fourier transform spectrometer, show variations of transmission in excess of 80% at 3.3 μm and over 20% at 4.7 μm, for a voltage range of 0-19 V. The suitability of this filter and related silicon micromachined components (microphones, accelerometers, etc.) for hydrocarbons and carbon monoxide domestic gas sensors is inferred.