Laboratory experiments are presented which show that liquid stratospheric aerosol droplets under polar winter conditions do not freeze for temperatures higher than the water ice saturation temperature (frost point). Calorimetric measurements of the freezing of supercooled H2SO4/HNO3/H2O bulk solutions with concentrations typical of the polar stratospheric aerosol exhibit very small freezing rates, which excludes the possibility of homogeneous freezing of the droplets for temperatures above the frost point. Even heterogeneous formation of H2SO4 and HNO3 hydrates at these temperatures is a very inefficient process unless the stratosphere offers nuclei better suited for nucleation than those present in the laboratory experiments, which appears to be unlikely. Only ice was found to be a potential nucleus suited for the formation of the hydrates, which could cause the hydrates to freeze at temperatures below the frost point. ¿ American Geophysical Union 1995