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An isotope exchange experiment with the short-lived radioactive tracer ¹³N is used to trace N₂O₅ uptake into nitrate-containing aqueous aerosol particles. While uptake of ¹³N-labelled N₂O₅ to deliquesced Na₂SO₄ aerosol is consistent with previous studies, efficient exchange of labelled nitrate with the non-labelled nitrate pool was observed in the presence of aerosol-phase nitrate. The experiments provide direct evidence for efficient bulk mass accommodation of N₂O₅ into aqueous solution with <i>α</i>_b &gt; 0.4 at room temperature, as well as for the fast dissociation into nitronium and nitrate. While for experimental reasons this study is limited to non-acidic nitrate-containing aerosol, it is likely that bulk mass accommodation and dissociation are not limiting N₂O₅ uptake under wider ranges of conditions.