There is extensive evidence to suggest that stratospheric aerosols can remain liquid to very low stratospheric temperatures, despite being highly supercooled. Even polar stratospheric clouds, which are a key factor in the depletion of ozone in polar regions, can often consist of liquid rather than solid particles. It has been known since the 1960s that stratospheric aerosols are mostly concentrated sulfuric acid-water droplets, but the combination of recent laboratory measurements, field observations, and thermodynamic model calculations has led to a recognition that many species other than water vapor can partition into the aerosols, particularly at low temperatures. This has been shown to increase the aerosol size, to control their freezing properties, and to affect the rates of important liquid phase reactions. This in turn influences the formation of polar stratospheric clouds and the subsequent extent and duration of seasonal ozone depletion in the polar regions. We review thermodynamic models of the liquid phase that enable the partitioning of gases such as HCl, HBr, HOCl, and HNO3 into sulfuric acid aerosols to be calculated over the full range of stratospheric conditions. Such models have been used to show that the uptake of nitric acid vapor can lead to a rapid transition from mainly sulfuric-acid- to mainly nitric-acid-based liquid aerosols at low temperatures, a process that has changed our view of how polar stratospheric clouds form. Liquid aerosol composition at these low temperatures is still known largely from predictions made by thermodynamic models, rather than from observations, and even laboratory data under these conditions are limited. This and other uncertainties in calculated aerosol composition are estimated, and their effect on the interpretation of particle observations and predictions made by chemical stratospheric models is described.¿ 1997 American Geophysical Union