Atmospheric organic aerosol is complex and thus a challenge to model. We introduce a theoretical framework (solubility distributions) to represent the solubility of multicomponent mixtures. Using the framework, we evaluate the commonly made assumptions about the cloud condensation nucleus (CCN) activity of organic mixtures. We find that material with water solubilities larger than 0.1-100 g/L can usually be treated as completely soluble, which simplifies the treatment of organic CCN.