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that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for highprecision 17 O measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-currenttuned cavity resonance to achieve reduced measurement drift compared with previous-generation instruments. Liquid water and water-vapor samples can be analyzed with a better than 8 per meg precision for 17 O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of 17 O are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of δ 18 O, δ 17 O and δD with precision of < 0.03 ‰, < 0.02 and < 0.2 ‰, respectively, based on repeated calibrated measurements.