Complex refractive indices have been retrieved directly from the FT-IR extinction spectra of laboratory-generated, binary sulfuric acid/water aerosols over a range of temperatures and compositions relevant to the atmosphere of Earth. The refractive indices are strongly dependent upon both temperature and composition, due to changes in the equilibrium between sulfate and bisulfate ions and the resultant change in the corresponding solvation interactions. The present results have important implications for determining the composition of sulfuric acid aerosols by remote sensing applications. ¿ 1998 American Geophysical Union