Surface-active organic compounds are common constituents of atmospheric aerosol particles. In this work, the effects of organic coatings on the heterogeneous hydrolysis of N2O5 was investigated. For this purpose aqueous NH4HSO4 particles were coated in situ in a large aerosol chamber. The experiments show that films composed of the ozonolysis products of α-pinene can reduce the reaction probability by more than an order of magnitude. Model calculations indicate that the main cause for the retardation of the hydrolysis is a slower uptake possibly due to a lower solubility of N2O5 in the organic film.