Complex refractive indices have been retrieved for the first time directly from the Fourier transform infrared extinction spectra of supercooled liquid HNO3/H2O aerosols at 220 K. Aerosol compositions were determined by simultaneously monitoring equilibrium HNO3 and H2O vapor pressures with a tunable diode laser. The new optical constants are compared with previous results from room temperature bulk solutions and <160 K amorphous solid thin films, and the differences are explained primarily in terms of the temperature-dependent dissociation of molecular HNO3 into its constituent ions. Mie scattering analysis of aerosol extinction spectra suggests that using either the room temperature liquid or the amorphous solid refractive indices may result in inaccurate particle size determination. These new data sets are directly applicable to the characterization of polar stratospheric clouds through spectroscopic remote sensing. ¿ 1999 American Geophysical Union