The uptake coefficients for N2O5 hydrolysis have been measured at room temperature on micron-sized aerosols composed of both sulfuric acid aqueous solutions and ammonium sulfate solutions. The measurements have been performed in a laminar flow tube which is coupled to a chemical ionization mass spectrometer for monitoring the concentration of N2O5 in the gas phase and an optical particle counter which sizes the aerosols and determines their number density. The aerosols are generated with an ultrasonic nebulizer from aqueous solutions of either sulfuric acid or ammonium sulfate, and their liquid-phase concentration is determined by the relative humidity (RH) set within the flow tube. For both the sulfuric acid and ammonium sulfate aerosols, the reaction probability (gamma) is largest for the lowest relative humidities studied : for sulfuric acid aerosols, gamma = 0.05-0.06 for RH = 9-20%, gamma = 0.02 for RH = 90%; for ammonium sulfate aerosols, gamma = 0.04-0.05 for RH = 50-69%, gamma = 0.02 for RH = 83-94%. In the case of ammonium sulfate aerosols, significant reactivity with N2O5 is found at relative humidities below the deliquescence point of 80%, consistent with the observation that ammonium sulfate aerosols are readily formed in supersaturated, liquid states.