The heterogeneous uptake of gaseous N2O5 by ammonium sulfate [(NH4)(2)SO4], ammonium bisulfate [NH4HSO4], and sulfuric acid [H2SO4] aerosols as a function of relative humidity has been investigated at room temperature and atmospheric pressure. Ammonium-containing aerosols were generated by a constant-output atomizer and conditioned by passing through a diffusion dryer. Sulfuric acid aerosols were produced by the homogeneous reaction of SO3 and H2O in a borosilicate vessel. Addition of a dry or wet N-2 flow controlled the relative humidity (RH) of these aerosol flows. Using a chemical ionization mass spectrometer (CIMS) for N2O5 concentration monitoring and a scanning mobility particle spectrometer (SMPS) for aerosol characterization, reaction probabilities (gamma) in the range of 0.001 to 0.1 for the uptake of N2O5 were determined as a function of RH. The results are expressed as follows: gamma[(NH4)(2)SO4] = 2.79 x 10(-4) + 1.30 x 10(-4) x (RH) -3.43 x 10(-6) x (RH)(2) + 7.52 x 10(-8) x (RH)(3), gamma [NH4HSO4] = 2.07 x 10(-3) - 1.48 x 10(-4) x (RH) + 8.26 x 10(-6) x (RH)(2), and gamma [H2SO4] = 0.052 - 2.79 x 10(-4) x (RH). We suggest that the water content and phase in the ammonium-containing aerosols control the reactivity of N2O5 while liquid-phase ionic reactions primarily dominate the uptake in sulfuric acid aerosols.