The infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped zirconia single crystals and undoped tetragonal zirconia powders, respectively. In the latter case, the reflectance spectrum was analyzed by an effective medium Theory, which has been proven to successfully determine the dielectric properties of small powder particles. The symmetry of the resulting modes of tetragonal zirconia was elucidated by performing theoretical calculations of the vibrational properties. ©1996 American Institute of Physics.