The CO(2)-H(2)O isotopic equilibration technique was studied for the routine analyses of the oxygen isotopic ratios of hot spring water. A reaction vessel containing 2 ml of water and 0.16 m mole of tank CO(2) was shaken for 18 hrs. in a constant-temperature bath at 25.0℃ (Figs. 1, 2, and 3), and the CO(2) was analyzed for the oxygen isotopic ratio by a MCKINNEY type mass spectrometer. Several aliquots of 1, 2 and 5 ml from a same water were each analyzed three times by successive equilibration (Table 1). The observed values differ depending on the volume of water but the corrected values by equation (6) indicate excellent agreement, implying the whole processes to be well controlled. The reproducibility of the isotopic analyses is better than ± 0.1‰ (Table 2) in most cases, and the accuracy would not be worse than ± 0.2‰ as demonstrated by the interlaboratory comparison of some standard samples (Table 3). Oxygen isotopic ratios of water from more than 70 hot springs in Japan are presented (Table 5). Although the results will be discussed in the following issues of this series of paper, most hot spring water have the δ(18)O values similar to those of meteoric waters in Japan. However, spring water from Arima Hot Springs, Hyogo-Pref., which has been known by its abnormally high chloride and low sulfate concentrations is of an exceptionally high δ(18)O value.