[en] A vacuum apparatus and the working procedure are described for the preparation of hydrogen by reduction of water with uranium, which is used for the determination of the isotopic D/H ratio in the range of natural concentrations of deuterium. A 25 to 40 nm³ sample of water is taken with a microsyringe and the reduction of water is carried out by circulating the water vapours over spongy uranium at 650 degC. The hydrogen formed is bound at 8O degC to additional uranium in the form of hydride which at 600 degC sets hydrogen free quantitatively by subsequent thermal decomposition. Three ampoules are then filled with released hydrogen at 80 kPa pressure, which are used for isotopic measurements in a mass spectrometer. The apparatus and the working procedure for hydrogen preparation were checked by a series of control analyses of reference samples. (author)