Fourier transform infrared transmission (FTIR) measurements of thin films of aqueous sulfuric and nitric acid and their ternary mixture were performed over a wide range of composition and temperature. On the basis of a systematic study of the spectra a database of optical constants for sulfuric acid (c less than or equal to 80 wt %) and nitric acid (c less than or equal to 50 wt %) for temperatures 183 less than or equal to T less than or equal to 293 K were obtained. A mixing rule is presented to calculate k((v) over bar) and n((v) over bar) spectra for H2SO4/HNO3/H2O solutions and validated against the new FTIR measurements of ternary solutions. This covers optical constants in the entire range of atmospheric relevance and is easily accessible for various applications, e.g., satellite aerosol observations.