Rationale Oxygen isotope ratio measurements of NO2- and NO3- by the azide method and denitrifier method are sensitive to the delta O-18 value of the sample water. However, the influence of delta O-18(H2O) on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of delta O-18(H2O) of a sample on the delta O-18 analysis of NO2- and NO3-.Methods We prepared NO2- and NO3- standards (with known delta O-18(NO2-) and delta O-18(NO3-) values) dissolved in waters having different delta O-18(H2O) values (delta O-18(H2O) = -12.6, 25.9, 56.7, and 110.1 parts per thousand). Nitrite and nitrate were converted into N2O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N2O were measured with a Sercon purge-and-trap gas chromatography/isotope ratio mass spectrometry (PT-GC/IRMS) system. The measured delta O-18 values of the produced N2O were plotted against known delta O-18(NO2-) and delta O-18(NO3-) values to evaluate the influence of exchange of an oxygen atom with H2O during the conversion of NO2- into N2O and NO3- into N2O, respectively.Results The degree of oxygen isotope exchange was 10.8 +/- 0.3% in the azide method and 5.5 +/- 1.0% in the denitrifier method, indicating that the azide method is more susceptible to artifacts arising from differences in the delta O-18(H2O) value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in delta O-18(H2O). Abiotic NO2-H2O equilibrium isotope effect experiments yielded a rate constant of (1.13 +/- 007) x 10(-2) (h(-1)) and an equilibrium isotope effect of 11.9 +/- 0.1 parts per thousand under the condition of pH = 7.5, 30 degrees C, and 2.5% salinity.Conclusions Oxygen isotope ratio measurements of NO2- by the azide method are highly sensitive to delta O-18(H2O) as a result of significant oxygen isotope exchange between NO2- and H2O. Therefore, to obtain the most accurate measurements water with the same delta O-18(H2O) value as that of the sample must be used to make the NO2- and NO3- standards.