A variable angle-of-incidence spectroscopic fixed-polarizer, rotating-polarizer, fixed-analyzer ellipsometer ~PRPSE! across a spectral range from 300 to 800 nm is used to determine the optical properties of anisotropic uniaxial tetragonal red mercuric iodide ~HgI2!. For the first time, to our knowledge, the bulk crystal HgI2 surface measured by ellipsometry was not subjected to potassium iodide cutting or etching. Measurements were made at an air–HgI2 interface with the optic axis parallel to the sample surface. To determine the optical constants, we varied both the angle of incidence and the azimuth of the optic axis with the plane of incidence. The detailed formulas needed for reliable procedures for analyzing the data are presented. The ordinary and extraordinary complex indices of refraction, ~no 2 iko! and ~ne 2 ike!, respectively, are determined. Good agreement between PRPSE and the prism technique for the refractive index is observed. The surface aging effects of the ellipsometric parameters of HgI2, during 30 h of exposure to air, were detected by PRPSE.