The partial pressures of Hg, Cd, and are obtained along the three‐phase curves for , solid solutions by measuring the optical absorbance of the coexisting vapor between 220 and 700 nm. The pressure ranges covered are 10−2–32 atm for Hg, 10−8–10−5atm for Cd, and 10−5–10−2 atm for , and Hg is always the predominant vapor species. The data allow the calculation of the chemical potentials of and and the Gibbs free enthalpy of formation for the solid solutions. In addition, the solidus temperatures are obtained directly from the optical absorbance measurements.