Optical absorption spectra of the vapor phase over HgI2(s,1) were measured for wavelengths between 200 and 600 nm at sample temperatures between 349 and 610 K. The spectra show that the samples sublimed congruently into HgI2 with no Hg or I-2 absorption spectrum observed. The Beer's Law constants for 15 wavelengths between 200 and 440 nm were determined. From these constants the vapor pressure of HgI2, P, was established as a function of temperature for the liquid and the solid beta-phases: In P(atm) = -7,700/T (K) + 12.462 (liquid phase), In P(atm) = - 10,150/ T (K) + 17.026 (beta-phase). The expressions correspond to the enthalpies of vaporization and sublimation of 15.30 and 20.17 kcal/mole, respectively, for the liquid and the beta-phase HgI2. The difference in the enthalpies gives an enthalpy of fusion of 4.87 kcal/mole, and the intersection of the two expressions gives a melting point of 537 K. Published by Elsevier Science B.V.