Atmospheric aerosols have mixed chemical composition, with a variety of inorganic (e.g., sulfate, nitrate, ammonium, and sodium) and organic species often present in a single particle. In the present study, we investigate experimentally the cloud condensation nuclei (CCN) activation of submicron aerosol consisting of an inorganic core (e.g., ammonium sulfate) coated by an organic film, at typical atmospheric supersaturations. We use two types of organic coatings on the (NH4)2SO4 particles. The first is glutaric acid, a CCN active organic found in the atmosphere, and the second species is dioctylphthalate (DOP), a nonhygroscopic organic. The CCN activation of (NH4)2SO4-glutaric acid particles was measured at a supersaturation of 0.3%, for different inorganic core sizes and organic film thickness. We found that a coating of glutaric acid increases the CCN activation of an (NH4)2SO4 particle and that this behavior can be predicted by K¿hler theory. The deviation from K¿hler theory for the mixed aerosol was determined by comparing theoretical and experimental CCN activation diameters for the particles and was found to be within experimental error. A thick coating of DOP (at least 70% by mass) did not hinder the activation of (NH4)2SO4 particles at supersaturations of 0.5 and 1.0%. The values for the measured activation diameters for the DOP coated (NH4)2SO4 particles were within the experimental error determined by the pure inorganic experiments, indicating that DOP was most likely acting as inert mass during activation. ¿ 1998 American Geophysical Union