The partial pressures of mercury, zinc, and tellurium will be determined for compositions of the solid solutions formed between mercury and zinc tellurides between 300 and 1000 C. This will be accomplished by measuring the optical absorbance between 200 and 700 nanometers of the vapor phase coexisting with a solid or liquid specimen in a sealed cell of fused silica. The chemical potentials and melting temperatures, as well as other thermodynamic properties, will be determined. The data are not available in the literature, and are needed in the processing of these technologically important materials, which are being used or considered for application in infrared detectors. The data will be important in guiding the efforts in crystal growth and subsequent annealing to control defect concentrations, and subsequently the electronic and optical properties.