The analyses of both O and N isotopic compositions of nitrate have many potential applications in studies of nitrate sources and reactions in hydrology, oceanography, and atmospheric chemistry, but simple and precise methods for these analyses have yet to be developed. Testing of a new method involving reaction of potassium nitrate with catalyzed graphite (C + Pd + Au) at 520 °C resulted in quantitative recovery of N and O from nitrate as free CO₂, K₂CO₃, and N₂. The δ¹⁸O values of nitrate reference materials were obtained by analyzing both the CO₂ and K₂CO₃ from catalyzed graphite combustion. Provisional values of δ¹⁸O_VSMOW for the internationally distributed KNO₃ reference materials IAEA-N3 and USGS-32 were both equal to +22.7 ± 0.5‰. Because the fraction of free CO₂ and the isotopic fractionation factor between CO₂ and K₂CO₃ were constant in the combustion products, the δ¹⁸O value of KNO₃ could be calculated from measurements of the δ¹⁸O of free CO₂. Thus, δ¹⁸O_KNO3 = aδ¹⁸O_free CO2 − b, where a and b were equal to 0.9967 and 3.3, respectively, for the specific conditions of the experiments. The catalyzed graphite combustion method can be used to determine δ¹⁸O of KNO₃ from measurements of δ¹⁸O of free CO₂ with reproducibility on the order of ±0.2‰ or better if local reference materials are prepared and analyzed with the samples. Reproducibility of δ¹⁵N was ±0.1‰ after trace amounts of CO were removed.