Rationale Measurements of oxygen-17 excess (17Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ17O and 17Oexcess values for NBS19 and NBS18, two international standards, for which δ18O values are already widely used as the references for carbonates. Methods The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ17O and δ18O values of this O2 allow δ17O values of CO2 to be obtained with very high precision (0.01 to 0.03 ‰) and, correspondingly, accurate 17Oexcess values with a precision of 5 per meg. Results We measured, for the first time, the δ17O values and 17Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015 ‰ and −227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041 ‰; 9.253 ± 0.018 ‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW. Conclusions Accurate values of δ17O and 17Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]