We present here the first measurements of the infrared complex refractive indices for supercooled ternary solutions of H2SO4, HNO3, and H2O under stratospheric conditions. The data sets were retrieved directly from the infrared extinction spectra of laboratory-generated aerosols. Comparisons of these results with a previously reported empirical mixing rule model(1) reveal significant differences, which we trace to errors in the binary acid/water thin film refractive indices used in the model. In addition, the model does not properly account for the concentrations of ionic and neutral species in the ternary solutions. The experimentally determined optical constants reported here will be useful for determining polar stratospheric cloud aerosol properties from infrared spectroscopic remote sensing measurements.