The ubiquitous presence of organic compounds in tropospheric particles requires that their role in aerosol/cloud interactions be accounted for in climate models. In this paper, we present studies that investigate the hygroscopic behavior of organic compounds and their efficiency as ice nuclei. Specifically, results for soluble and partially soluble dicarboxylic acids that have been observed in atmospheric aerosol are discussed. At room temperature, we use a humidified tandem differential mobility analyzer (HTDMA) and a condensation particle counter interfaced with a cloud condensation nuclei counter to characterize the water uptake behavior of these acids. The HTDMA data agree quite well with modeled hygroscopic behavior. However, we find that some of the compounds retain water to very low humidities, never exhibiting efflorescence. The studies are extended to lower temperatures using a continuous flow ice thermal diffusion chamber to investigate the role of these species in ice nucleation at cirrus condi...