The invention is related to the instrumental analytical chemistry, further more to the stable isotope determination in food products. The present invention is referred to the alcohol thermal dehydration chamber, apparatus for “online” determination of isotopic composition of non-exchangeable hydrogen stable isotopes in ethanol samples, which comprises of: A) Alcohol thermal dehydration chamber, B) Detection device, which contains pyrolysis reactor and the Continuous Flow Isotope Spectrometer, and it is connected to the alcohol thermal dehydration chamber over C) System of valves, connectors and capillary tubes which are used for transfer of analyzed sample as for the purging of the alcohol thermal dehydration chamber; procedure for “online” determination of isotopic composition of non-exchangeable hydrogen stable isotopes in ethanol samples and procedure for “offline” preparation of ethene (ethylene) gas by means of alcohol thermal dehydration chamber and for the purpose of
