Knudsen cell mass spectrometry has been applied to study the quasi-binary system CdTe-ZnTe at 900 K. Composition of the vapors, vapor pressures, activities, and free energy of mixing were determined. Activities of CdTe were shown to have negative deviation from ideal behavior, whereas those of ZnTe exhibited positive deviation. Free energy of mixing in CdTe-ZnTe is more negative than that of the ideal solution. Pressure-composition P-X section of the Pressure-temperature-composition P-T-X phase diagram was constructed. and relationship between the composition of the crystal, vapor pressure and composition of the vapor was determined in the whole concentration range. At 900K. CdTe-ZnTe was found to be a continuous solid solution with no phase separation and without congruent sublimation. (C) 2002 Elsevier Science B.V. All rights reserved.