The invention relates to a method and apparatus for determining the concentration of at least carbon dioxide in a gas mixture on the basis of the absorption of infrared radiation (2), said apparatus comprising: a radiation source (1), the radiation emitted thereby being aligned to travel through a measuring cell (4) containing a gas mixture (6) to be analyzed; an optical band-pass filter (10), which is transmissive to a first wavelength band and positioned on the path of radiation emerging from the measuring cell or entering into the measuring cell through which the radiation passes; and a first detector (14), positioned in the radiating direction downstream of said first band-pass filter and used for detecting the radiation intensity falling thereon. Said optical band-pass filter (10) has a band-pass which lies within the wavelength range of 4.38 .mu.m-4.47 .mu.m for measuring the total concentration of carbon dioxide by means of the absorption spectrum of a molecule .sup.13 CO.sub.2 produced by a carbon isotope .sup.13 C.