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The heterogeneous reaction of N₂O₅ on mixed organic-inorganic aerosol particles was investigated using an entrained aerosol flow tube coupled to a custom-built chemical ionization mass spectrometer. Laboratory results on aqueous particles confirm a strong dependence of the reactive uptake coefficient (γ) on particle liquid water, for particle water concentrations below 15 M, and the molar ratio of particle water to nitrate. Measurements of γ (N₂O₅) on mixed chloride-nitrate particles indicate that the presence of trace chloride can negate the suppression of γ(N₂O₅) at high nitrate loadings with implications for polluted coastal regions. These results are used to construct a new parameterization for γ (N₂O₅), that when coupled to an aerosol thermodynamics model, can be used within regional and/or global chemical transport models.