Abstract: A measuring device for determining the concentration of gases with two identical radiation sources and two radiation detectors, which delivers stable measured values despite the contamination that occurs or the shielding from radiation of the outer optical surfaces exposed to the gases or to mixtures of gases and despite possible mechanical disadjustments. The two radiation detectors are provided with an optical concentrator each for bundling the radiation, and being arranged in a gas-tight housing together with the two identical radiation sources and with a beam splitter. One radiation source is directed through a window that is transparent to infrared light to a plane mirror outside the gas-tight housing, and the beam path reflected by the plane mirror falls through the window that is transparent to infrared light onto the beam splitter.