The automatic sample preparation system for oxygen isotope analysis of natural water samples was constructed. The system is essentially a modification of that originally designed by W. Dansgaard in the University of Copenhagen. Sixty water samples of 5 ml each are automatically equilibrated with CO(2) of 30 ml NTP each within 4.5 hours. The equilibrated CO(2) gases are successively measured for their oxygen isotopic ratios by an automatic mas-sspectrometer, VG903. The time required for the
measurement of the 60 CO(2) samples is about 15 hours; thus approximately 20 hours are required to complete the isotopic measurements of 60 water samples. The accuracy of the oxygen isotopic analyses is about 0.1%. The result of oxygen isotopic analyses by the automatic preparation system agrees with that of the conventional method within about +0.1%. The results indicate that the automatic preparation system is applicable for the oxygen isotope analysis of natural water samples.