The effect of pH of water on the measurement of oxygen isotopic ratio by means of CO(2)-H(2)O isotopic exchange technique was studied. In this study, a new type of reaction vessel was used (Fig. 1). The reaction time for equilibration using this new reaction vessel was nearly equal to that in the previous study (Fig. 2). The oxygen isotopic ratios of each two samples of pH 2, 4, 7, 8, 9, and 10 were measured. There is no significant difference between the observed δ(18)O values of these samples (Table 1). Therefore it is concluded that the effect of pH of water needs not to be taken into account on the measurement of oxygen isotopic ratio by means of CO(2)-H(2)O isotopic exchange technique.