Results of isotope investigations of atmospheric condensate and potable water supplied from Earth confirmed the fractionating of hydrogen and oxygen stable isotopes in the organism of ISS crewmembers. Samples of ISS atmospheric condensate were characterized by altered concentrations of 18O2 and deuterium. Atmospheric deuterium quantities varied in the interval of -67 to 79 +/- 3 ppm, whereas water deuterium made up -85 +/- 3 ppm as compared with SMOW. Atmospheric condensate consists of water vapor transpired and perspired by humans. In addition, cells of crews' red blood were also found to contain increased concentrations of deuterium as compared with the deuterium content of potable water. Results of isotopes measurements in urine, blood, saliva and potable water in a chamber experiment also evidenced in favour of the body fractionating of the stable isotopes of hydrogen and oxygen.