The ionic speciation and the optical properties of aqueous H2SO4 have been investigated as a function of temperature and acid concentration by spectroscopic techniques. FT-Raman spectra have been obtained of 1.43-44.04 molal (12-81 wt %) aqueous H2SO4 in the temperature region from 300 to 220 K. The degree of dissociation of the second dissociation step in aqueous H2SO4 (alpha(2): HSO4- reversible arrow H+ + SO42-) is derived from the relative intensities of the HSO4- Raman bands around 1050 cm(-1) versus the SO42- band around 980 cm(-1), and a polynomial parametrization of alpha(2) is presented. FT-IR specular reflectance spectra are obtained of 6.46-44.04 molal (38-81 wt %) aqueous H2SO4 in the temperature region from 300 K to as far down as possible before crystallization by freezing occurred. The complex index of refraction is obtained from the IR reflectance spectra by the use of the Kramers-Kronig transformation, and the importance of including far-infrared data in the transformation is demonstrated. A revised parametrization of the density of aqueous sulfuric acid is given and an interpolation algorithm for obtaining the complex index of refraction of aqueous sulfuric acid solutions as a function of acid weight fraction and temperature is presented.