The ability of partially soluble organic species to act as cloud condensation nuclei (CCN) has been studied. A K¿hler model incorporating solute solubility and droplet surface tension describes the behavior of solid adipic and succinic acid particles, whereas solid azelaic acid activates much more efficiently that predicted. In addition, it was shown that trace levels of either sulfate or surface active species have a dramatic effect on the activation of adipic acid, a moderately soluble organic, as predicted by the full K¿hler model. For internally mixed particles in the atmosphere, these effects will greatly enhance the role of organic aerosols as CCN.