In principle, ionic liquids (ILs) can serve as stable and less harmful high-refractive-index liquids (n>1.60). However, detailed information on their complex refractive indices and dispersions is rare. Therefore, we determined the optical constants of six commercially available low-index ILs (n ca. 1.4) and two high-index ILs (n max. 2.38) synthesized by us utilizing spectroscopic ellipsometry. Furthermore, we tested the ILs as immersion liquids for different photonic crystals. Analysis of the optical properties of these photonic crystals after infiltration demonstrates potential applications of ILs in optofluidics and as index-matching liquids.