A thermodynamic model is employed to investigate the effects of cooling sulfuric acid tetrahydrate (SAT) when the polar stratosphere is perturbed by denitrification or volcanic eruptions. Also, survey experiments are carried out to study the plausibility of the predictions of this model. In contrast to an atmosphere rich in HNO3 for which Koop and Carslaw [1996> calculated that SAT deliquesces upon cooling to form a ternary solution, in a denitrified atmosphere the liquid begins to form ~3 K colder, and SAT is not predicted to deliquesce completely above the ice frost point. Likewise, in an atmosphere with a high sulfate loading, solid/liquid aerosols are predicted to coexist over a large temperature range. For these two cases, SAT particles should thus deliquesce only partially upon cooling, and hence, should return to solid form upon warming. Furthermore, the results of laboratory experiments suggest that HNO3-induced deliquescence of SAT might be too slow to matter even under normal stratospheric conditions. ¿ 1998 American Geophysical Union