The glass formation and crystallization of GeSe2-Sb2Te3 molten alloys is analysed from the results of differential scanning calorimetry and X-ray diffraction. The phase diagram of this quasi-binary system is calculated and compared with the experimental results. An ideal associated solution model is used for the liquid mixtures and a substitutional solution model for the Sb2Te2(GeSe2) solid phase. Gibbs free energy curves for the supercooled liquid relative to the stable crystalline phases are constructed. The success of these curves to describe the glass forming ability in this system is discussed. © 1985.