A method is described for continuous measurement of the concentration of a polyatomic non-elementary gas using a single-beam infrared photometer. Alternating radiation originating from a modulated radiation source simultaneously strikes two radiation detectors, after passing through an absorption cuvette and two subsequent interference filters, one detector being used to form a reference signal. The transmission range of one of the interference filters coincides with the absorption maximum of one of the specific absorption bands of the gas whose concentration is intended to be measured. The transmission range of the other interference filter is at another wavelength band in which the gas to be measured absorbs no radiation. The output signal of the radiation detector which forms the reference signal is used to adjust the radiation source such that the reference signal always has the same value when the ambient temperature changes or the radiator changes and the change thus obtained in the adjustment quantity for the radiation source is used to form a signal which, by means of a correction circuit and a division circuit, keeps the output value of the division circuit constant when the ambient temperature changes or the radiator changes.
