Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO₃⁻ in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying δ¹⁸O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO₃) with low δ¹⁸O and USGS35 (NaNO₃) with high δ¹⁸O and ‘mass-independent’ δ¹⁷O. The procedure used to produce USGS34 involved equilibration of HNO₃ with ¹⁸O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO₃⁻ reference materials with a range of δ¹⁸O values and normal (mass-dependent) ¹⁸O:¹⁷O:¹⁶O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (α) between [NO₃⁻] and H₂O decreases with increasing temperature from 1.0215 at 22°C to 1.0131 at 100°C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high ¹⁷O:¹⁸O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO₃⁻ isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO₃⁻ samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of ±0.2–0.3‰, 1σ): IAEA-N3 has δ¹⁸O = +25.6‰ and δ¹⁷O = +13.2‰; USGS32 has δ¹⁸O = +25.7‰; USGS34 has δ¹⁸O = −27.9‰ and δ¹⁷O = −14.8‰; and USGS35 has δ¹⁸O = +57.5‰ and δ¹⁷O = +51.5‰.