We present a significantly improved set of complex indices of refraction ( optical constants) for crystalline ( NH4)(2)SO4 at 298 K, determined from extinction spectra measured in an aerosol flow tube ( AFT). The improved values provide more accurate reproductions of experimental extinction spectra when used in light scattering calculations ( Mie, T-matrix, etc.). Optical constants were also derived from measurements using a cryogenic AFT at 243, 223, and 213 K, temperatures characteristic of the upper troposphere and stratosphere. Only minor changes in the optical constants were observed down to 223 K, the transition temperature to the ferroelectric phase, after which significant changes were observed. Here we report the first complex indices of refraction at reduced temperatures for both phases of crystalline ( NH4)(2)SO4.