A method and an arrangement for measuring the concentration of gases in a mple includes the generation of a monochromatic light beam having predetermined color characteristic. An indicator generates light signals indicative of the concentration of the gases in a sample to be measured and includes a light-transmissive surface positioned to be impinged by the monochromatic light beam, a diffusion membrane adapted to be placed in the proximity of a sample and being permeable to a selected gas component thereof, and an indicating substance positioned to be impinged by the monochromatic light beam penetrating the light-transmissive surface and by the gas component penetrating the diffusion membrane. The indicating substance reacts when illuminated by the incident monochromatic light by emitting a resultant light beam having an emitted component which has a color characteristic different from the predetermined color characteristic of the monochromatic light beam. The resultant light beam is conducted away from the indicating substance through the light-transmissive surface. Finally, the emitted component is discriminated from the resultant light beam so that the change in the color characteristic of the indicating substance can be measured and correlated with the concentration of gases in the sample.