The kinetics of the hydrolysis reaction of N2O5 on secondary organic aerosol (SOA) produced through the ozonolysis of alpha-pinene and on mixed ammonium bisulfate-SOA particles was investigated using an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. We report room temperature uptake coefficients, gamma, on ammonium bisulfate and SOA particles at 50% relative humidity of 1.5 x 10(-2) +/- 1.5 x 10(-3) and 1.5 x 10(-4) +/- 2 x 10(-5), respectively. For the mixed ammonium bisulfate-SOA particles, gamma decreased from 2.6 x 10(-3) +/- 4 x 10(-4) to 3.0 x 10(-4) +/- 3 x 10(-5) as the SOA mass fraction increased from 9 to 79, indicating a strong suppression in gamma with the addition of organic material. There is an order-of-magnitude reduction in the uptake coefficient with the smallest amount of SOA material present and smaller additional reductions with increasing aerosol organic content. This newly coated organic layer may either decrease the mass accommodation coefficient of N2O5 onto the particle or hinder the dissolution and diffusion of N2O5 into the remainder of the aerosol after it has been accommodated onto the surface. The former corresponds to a surface effect and the latter to bulk processes. The low value of the uptake coefficient on pure SOA particles will likely make N2O5 hydrolysis insignificant on such an aerosol, but atmospheric chemistry models need to account for the role that organics may play in suppressing the kinetics of this reaction on mixed organic-inorganic particles.