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3 ) plot with a slope of 0.526 ± 0.003 (standard error at 95% confidence limit), but the line for the waters (VSMOW, GISP, SLAP and MSA-6) seems to have a slightly steeper slope of 0.528 ± 0.001, although difference of the slopes is not statistically significant.Keywords: oxygen three-isotope ratios, silicate reference materials, VSMOW-SLAP normalization, terrestrial fractionation line, BrF 5 fluorination to CO 2 to compare its oxygen isotopic ratio with CO 2 that has been equilibrated with reference water of a known isotopic composition (such as VSMOW). In this comparison, the equilibrium fractionation factor α CO2-H2O comes into the calculation. Since the reported values of α CO2-H2O at 25°C range from 1.0407 to 1.0412 (Friedman and O'Neil, 1977;Chako et al., 2001), this variability causes serious uncertainty in δ18 O values of minerals relative to VSMOW, if the oxygen isotopic ratios are determined using CO 2 as an analytical gas for mass spectrometry. To avoid the ambiguity, direct comparison of O 2 liberated from reference water samples with that from minerals is preferable. In the present note, we show the results of oxygen isotopic ratios of some international reference minerals by direct comparison of oxygen isotopic ratios of VSMOW and SLAP. ANALITICAL PROCEDURESOxygen was extracted in the form of O 2 from silicate and oxide minerals using a laser fluorination system with BrF 5 as a reagent (Sharp, 1990). The laser used in our system was a CO 2 laser with a maximum power of 12W and the wavelength of 10.6 µm (Model JLC-1259, Japan Laser). The focal length was 10.2 cm and a minimum spot diameter was 100 µm. A visible, coaxial He-Ne laser (wavelength of 633 nm) was attached to locate a target. The technical details of the laser fluorination system have