Abstract. The oxygen isotope composition of nitrogen oxides (NO_x) in the atmosphere is a useful tool for understanding the oxidation of NO_x into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NO_x due to NO_x–O₂–O₃ photochemical cycling. At low NO_x / O₂ mixing ratios, NO_x became progressively and nearly equally enriched in ¹⁷O and ¹⁸O over time until it reached a steady state with Δ¹⁷O values of 39.3 ± 1.9‰ and δ¹⁸O values of 84.2 ± 4‰, relative to the isotopic composition of the initial O₂ gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O₂, and NO_x. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O₃ formation play a dominant role in controlling NO_x isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NO_x cycling. The data and model were consistent with previous studies which showed that the NO + O₃ reactions occur mainly via the transfer of the terminal atoms of O₃. The model predicts that under tropospheric concentrations of NO_x and O₃, the timescale of NO_x–O₃ isotopic equilibrium ranges from hours (for ppbv NO_x / O₂ mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ¹⁷O and δ¹⁸O values of 45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O₃ has high concentrations, the equilibrium between NO_x and O₃ should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NO_x conversion to HNO₃ is faster. The experimentally derived rate coefficients can be used to model the major NO_x–O₃ isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.