Abstract: Process of and apparatus for the simultaneous determination of the concentration of a plurality of radioactive gases. Radioactive gas containing several beta radionuclides is led into a separate chamber where their concentration is determined simultaneously and selectively by using a plastic multi-layer scintillator. The energy of the emitted beta radiation further identifies single radionuclides, and it determines them selectively and quantitatively by carrying out an amplitude discrimination of the registered impulses. The process can be carried out continuously or discontinuously. The multi-layer scintillation chamber is composed of a light-tight casing in which plates of plastic scintillators having suitable dimensions and location are placed in spaced relationship, such plates being connected on each end of the casing with a photomultiplier through a light-guide.