Laser-based spectroscopic techniques, such as cavity ring-down spectroscopy (CRDS), provide a new, costeffective and more widely available approach to measure the oxygen isotope ratio in water molecules, H218O/H216O (d18O), and are used increasingly to measure d18O in the world’s oceans. Here, we present results froman interlaboratory comparison designed to evaluate the quality of CRDS-derived measurements, and theirconsistency with values measured by isotope ratio mass spectrometry (IRMS). We also discuss the influenceof salt on instrument performance and sample throughput for the analysis of seawater samples. This studycompared measurements of d18O from natural samples with a wide range of salinities (0, 29.4, and 34.6) performedby four independent labs: two using CRDS and two using IRMS. We also compared d18O measurementsof Northeast Atlantic Deep Water collected in 2013, 2012, 2009, and 1995 from the AR7W repeathydrography transect across the Labrador Sea. The within-lab precision of ocean-based CRDS measurementsis seen to approach 0.03&, which is better than the manufacturer’s typically stated analytical precision(around 1/2 0.05&), and comparable to that achievable with IRMS. The interlaboratory differences of measurements(highest-lowest) reported by the four labs is taken as an indicator of overall accuracy, and is estimatedconservatively as being<0.1&, with the potential to approach 0.05&. Overall, these results show thatCRDS based 18O measurements of seawater can be equivalent to high-quality measurements by IRMS.