Interaction with ozone transfers its anomalous (non-mass-dependent) 17O enrichment to atmospheric nitrogen oxides and nitrate. The 17O anomaly (Δ17O) in nitrate can be used to identify atmospheric nitrate inputs into terrestrial and aquatic environments as well as to study the role of ozone in the atmosphere s reactive nitrogen cycle. We report here on an online method for analysis of the 17O anomaly, using a strain of denitrifiers to convert nitrate to N2O, which decomposes quantitatively to N2 and O2 in a gold furnace at 800 °C, followed by gas chromatographic separation and isotope analysis of O2. This method requires ∼50 nmol of nitrate, 2-3 orders of magnitude less than previous offline thermal decomposition methods to achieve a similar analytical precision of 0.5‰ for Δ17O. There is no significant memory effect, but calibration via nitrate or N2O reference materials is required for scale normalization. The N2O decomposition method is shown to be well-suited for nitrate analysis in freshwater and seawater samples from various environments. © 2007 American Chemical Society.