A method and apparatus for detecting minority gaseous species in a mixture by light-emission spectroscopy by means of an optical spectrometer (8), in which the radiation emitted by a plasma (4) present in the gas mixture for analysis is used and, in the spectrum of the radiation, lines are identified of a majority gaseous species that present amplitudes that are sensitive to the presence of a minority species, and information about the concentration of a minority gaseous species is deduced from the amplitude(s) of the sensitive line(s). This makes it possible to monitor minority gaseous species in real time.