Abstract: A gaseous mixture containing a number of molecular gases is analyzed to establish the amount of each individual gas by directing a light beam along a measurement path 11 through the gas. Each of the spectral component is selected to coincide with a respective vibrational-rotational-excitation band of one of the gases. The attenuation produced in each of the spectral components in its passage along the measurement path 11 serves as an indication of the concentration of the associated gas. The spectral components are generated either from distinct light sources 12, 13, 14 or are alternatively produced by spectral division of light from a multiple line or broad band source. Different marking frequencies f1, f2 and f3 are used to modulate each of the spectral components so that the signal received from the single photoelectric receiver 18 can be subsequently demodulated by three demodulators 19, 20, 21 to recover information relating to the individual spectral components.