Abstract The equilibrium temperature-composition ( T-x) phase diagram of the CdSeCdTe system was calculated theoretically. Condensation diagrams of “thick” (2–10 μm) and “thin” (0.1 μm) films of CdSe x Te 1− x solid solutions were constructed over wide ranges of substrate temperature (320–800 K) and of molecular flux density R +Σ of CdSe and CdTe vapours incident on the substrate ((2–200) × 10 22 m -2 s -1). It was shown by experiment that in the synthesis process at T s < 450 K thick films of CdSe x Te 1− x solid solutions undergo disintegration. The substrate temperature was found to be the main factor determining the ranges of isomorphous substitutions in CdSeCdTe films obtained under conditions close to equilibrium. The experimental data on the phase composition of CdSe x Te 1− x films are in quantitative agreement with the theoretically calculated binodal phase diagram of the CdSeCdTe system.