Abstract: The invention relates to an arrangement for measuring the concentration of gaseous and/or vaporous components of a fluid mixture by means of light absorption such as by means of an infrared absorption by utilizing a membrane which is made of a material in which the component of the fluid mixture to be measured accumulates. The membrane is transparent to the measuring radiation and is selectively permeable for the particles of the compounds to be measured. The membrane further acts on the radiation path between the light source and the detector. The arrangement is improved with respect to selectivity and the detection sensitivity. This is achieved in that the membrane is mounted in the flow path of the fluid mixture such that the flow path is subdivided into two component paths at the surfaces of the membrane.