We present a MEMS infrared spectrometer for selective and quantitative chemical gas analysis. infrared absorption spectroscopy can distinguish gases easily and also detect nonreactive molecules like CO2, in contrast to e.g. metal oxide gas sensors The following new spectrometer concept avoids expensive linear detectors as used for grating spectrometers: A tunable interference filter scans the desired part of the infrared spectrum. A single pixel thermopile detector measures serially the intensity at selected wavelengths. The tunable optical interference filter is fabricated by a new porous silicon batch technology using only two photolithography steps. The refractive index of this filter microplate is gradually modulated in depth to create a Bragg minor, edge filter or a Fabry-Perot bandpass filter for central wavelengths, between 400 nm and 8 mum. Two thermal bimorph micro- actuators tilt the plate by up to 90 degrees, changing the incidence angle of the beam to be analyzed. This tunes the wavelength transmitted to the detector. The filter area can be chosen between 0.27 mm x 0.70 mm and 2.50 mm x 3.00 mm, its thickness is typically 30 mum. The spectral finesse lambda/Delta lambda of 25 is sufficient for most diagnosis applications. First results showed that CO2 and CO can be detected selectively with this system - which is interesting in combustion processes - by measuring their absorption at 4.26 mum and 4.65 mum respectively. Other wavelength ranges e.g. for liquid analysis or colorimetry of visible light are possible.