Differential scanning calorimetry analysis was used to gain insight on the thermal stability and calorimetric behaviour on heating of the Ge25Sb20S55 glassy alloy. The as-quenched glass shows a glass transition at Tg=570 K. From the variation of Tg with the heating rate a value of Δh=240 kJ mol-1 was obtained for the activation enthalpy of the mean relaxation time. On further heating an exothermic process appears ranging from 725 to 800 K giving rise to glassy GeS2 and needle-like crystals of antimony. The microstructure of the crystallization product was obtained from X-ray diffraction and scanning electron microscopy. The resulting material shows two glass transitions. The crystallization process is explained correctly by the Johnson-Mehl-Avrami-Erofe'ev equation with a kinetic exponent of n=1.6. The apparent activation energy of crystallization is evaluated as E=286 kJ mol-1. © 1991 Chapman and Hall Ltd.