Recent developments in far infrared laser spectrometry and dispersive Fourier transform spectrometry have allowed the full spectral variation of both parts of the complex refractive indices of some aqueous salt solutions in the spectral region 25–450 cm−1 to be determined. The salts studied were LiCl, LiClO4, NaCl, NaClO4, NaI, KCl, KBr, KI, KF, MgCl2, Mg(ClO4) and Bu4NBr (tetrabutylammonium bromide). The results show that in the presence of some electrolytes the optical constants of the solution differ significantly from those of pure water, and that in localised spectral regions the absorption can be many times less than that of pure water.