An infrared radiation gas analyzer for determining the concentration of an ingredient in a sample gas has a sample gas container for containing a sample gas at a temperature at which the ingredient the concentration of which is to be determined will emit infrared radiation in a range characteristic of the ingredient and a window for allowing the infrared radiation to escape from the container. A pair of filters is provided, one of the filters transmitting only radiation in the range and the other of the filters transmitting only radiation in a range near to the firstmentioned range. A disc with the filters thereon rotates in front of the window for passing the filters across the path of radiation escaping from the container. An infrared radiation detector is positioned for receiving the radiation passed by the respective filters and emitting a pulsed signal, which is fed to a comparator for determining the difference between or the ratio of the respective pulses of the signal as an indication of the concentration of the ingredient the concentration of which is to be determined in the sample gas.