Aerosol particles found in the lower confines of the atmosphere are typically internal mixtures of sulfate, inorganic salts, refractory components, and organic species. The effect these complex combinations have on cloud formation processes remains largely unknown. We have conducted two complementary studies on one important process, the homogeneous formation of ice by small particles. In the first study the freezing of atmospheric aerosol was induced using controlled temperature and humidity conditions. In the second study the chemical composition of the residue from ice crystals in high altitude clouds was analyzed. Here we show that organic components do not partition equally to the ice and aqueous phases. Instead, organic-rich particles preferentially remain unfrozen. These results suggest that emissions of organic species have the potential to influence aerosol-cold cloud interactions and climate.